Word Problem with Percents - an example

November 12th, 2006

Below is a relatively easy word problem with percents. It was posed on a forum and sent to me by one of LearningByYourself.com members. The “relatively easy” term is because many find the question difficult to answer. Even if shown an answer they would find replication of the reasoning difficult. The key to understanding the difficulty in answering it is in the difference between static and operative thinking, and how to teach word problems. You can find more discussion of it in the two following links:
LearningByYourself.com

I first quote the forum discussion, which is a typical one. Note that one of the discussants is a math teacher. I then write my explanation of “how to explain”.

Questions or feedback can be sent to help@LearningByYourself.com

Q: At a concert, there were 20% more men than women. If there were 420 men, how many more men than women were there?

Can someone explain HOW to solve this one? Thanks!

*RESPONSE 1*

WOMEN: =====\=====\=====\=====\=====
MEN: =====\=====\=====\=====\=====\===== (not just 5 bars, like the women, but 20% more, or 6 bars)

TOTAL MEN BARS: 6 — so divide the number of men per number of MEN BARS. 420 divided by 6 equals 70. This tells us that each bar is worth 70 people.

Therefore, 70 times the 6 MEN BARS equals 420.

AND

70 times the 5 WOMEN BARS equals 350.

The difference is the value of one bar, or 70.
__________________

*Original Poster*

Alright, I follow you & it makes fine sense. *HOWEVER, how did you decide to set the women to 5 bars and the men to 6 bars?
*
This is what I did:
/————————–/——————/
60% men 40% women
So we tried to solve for 1% by dividing 420 by 60=7.
Then we said 7 x the 20% = 140 for the difference betw men & women.

Now, where did our reasoning go wrong?
_______________________________
*Responder 1*

ANSWER #1: I decided to make the women 5 bars because it is easy to find 20% of 5 — which is 1, and 5 plus 1 equals 6 which is why the men are 6 bars.

ANSWER #2: You went wrong when you said 60% MEN and 40% women. There is a difference of 20 between 60 and 40, yes. BUT — the question tells you there were 20% more men then women.

According to that, if you had 40% women, you’d need to take 20% of 40% (20% more men). 20% of 40% is 8%. So you’d have 40% women and 48% men — that doesn’t add up, so to speak. So you can kind of tell that the “60%/40%” thing isn’t going to work, just by looking at it that way, before actually working it out. Make sense? or, clear as mud, and you are looking at me with the glazed-eye look, of which my DS is so fond!?
__________________

*OP*

Thank you for your gentle explanation that doesn’t leave me feeling quite so stupid as I felt trying to work this with my ds this morning!
_______________________

*RESPONSE*
** My gentle explanation was born of a tiny *little moment of panic when you asked why’d you do it that way…. WHY?! I don’t know WHY!?!*

*RESPONSE FROM A SECOND PERSON*

My solution is a little different.

I thought of using the unknown x ( the number of women) and multiplying it by 1.2 (this adds the 20%) and making that equal to 420

so, 1.2x = 420

1.2x/1.2 = 420/1.2

x = 350

so x is 350, and this is the number of women. So therefore, we have 420 men and 350 women, so by subtracting, we find there are 70 more men than women.

To me, that’s easier. But maybe in Singapore 6B, you aren’t solving for unknowns yet? Am I right? We’re still in Singapore 4B….
__________________

*FIRST RESPONDER to SECOND RESPONDER*
Your answer is correct.

But the thing is, you used algebra. The student is supposed to solve the problem using the bar diagram technique.

Now, I will say that often as the teacher “I will work out the problem privately using algebra, and then will be able to suddenly ‘get’ how to explain the problem using bar diagrams to my student.”
** **

Explanation

Here is how to explain the problem. It would seem a bit long, but the idea is not to explain the solution, but how to get to it EVEN when you don’t know how to process the ideas but can follow translation rules, and assuming you studied the Percents book.

We start with the first sentence, “there were 20% more men than women”. We do not even read the second sentence until we figure this one out. The idea is to minimize information load.

Before translating we note that the issue is percentage.

Rule: Percents are ALWAYS relative to a whole (read my Percents book to understand it fully).

In the sentence the comparison is men vs. women. Therefore the whole is one of those.

Rule: The Whole is always the one the percentage is compared to.

Since the sentence says, “Men are 20% more than women” the percentage is compared to the women. Therefore the women are the “whole”.

Now we can rewrite the sentence a bit:

  • Men are 20% more than the women.
  • Women represent 100%

Translation to math is easy now:

  • Women = 100% = “whole”
  • Men = 100%+20%=120% compared to women –> *Men = 120%Women.

When dealing with percents we prefer to convert those to fractions or decimal fractions.

  • 120% of the whole = 1.2Whole

Result: 120%Women = 1.2Women

Note: we do not use bars or X&Y! Why use any such artificial construct? Men and Women are perfectly good variables and MUCH more intuitive than either bars or X&Y.

Now we are ready for sentence #2. Sentence #2 has two parts and should be broken to the two sentences it represents (sentence = idea in this context).

  • Sentence 2a: There were 420 men.

Note: breaking the sentence removes the “if“. For various reasons it is easier for us to process factual sentences compared to hypothetical sentences.
Now this sentence is easy to translate:

  • Men = 420

Rule: do what you can. We can take the first math sentence (a.k.a equation) and combine it with the second one we just wrote. We can see that “Men” is equal to two different things. We can therefore take 420, for example, and exchange the “Men” in the first sentence with it.
Result: 420=1.2Women
Note:
operative mode of thinking would often explain it as “if A = B and B= C then A=C”, which is a more difficult concept than just replacing B in the second sentence with the A from the first.

Rule: do what you can.
Rule: simplify.

We can divide both sides by 1.2 and that will simplify the sentence.
420/1.2 = Women

Rule: do what you can.

We can solve the left side.

420/1.2 = 350.

Therefore we have: 350 = Women.

We can also use this sentence with the Men=1.2Women sentence.

We get Men = 1.2×Women
Or Men=1.2×350=420, which we knew. This is good since we just checked our process by getting something we already knew to be true.

Now we can move to the second part of sentence 2. It reads “How many more men than women?”

Translation rule: more than = difference between the first “object” and the second “object”.

Therefore, the sentence is: What is the difference between Men and Women? [note, I changed men and women to the “math version” already].

Translation rule: difference means subtracting the second object from the first.

Result: Men - Women = the answer.

But we know that Men = 420 and Women=350.

We use the exchange rule and get:

  • 420-350=Answer
  • 70 = answer.

Writing the answer: we take the question sentence in the problem “How many more men than women were there?” and write it in the positive form with the answer attached.

Result: There were 70 more men than women.

In no place did I use “operative” mode of thinking because while using the “operative” mode makes it easier to understand the solution IF explained, it makes it much more difficult to learn how to generate the solution. The solution above is much more long-winded, but much easier to generate once you learn the translation rules, the problem solving rules and how to apply them.

About Math Standards

August 29th, 2006

Math Standards in the US. I was “happy” to see that what I’ve been saying is supported there. I urge you to read the report if you want to understand some of things you should do or don’t do with your kids. You can find the complete report, as well as the executive summary here http://www.edexcellence.net/foundation/publication/publication.cfm?id=338&pubsubid=1117#1117
Here is an excerpt from the executive summary, with my comments in italics:

Common Problems

Why do so many state mathematics standards come up short? Nine major problems are widespread.

1. Calculators

One of the most debilitating trends in current state math standards is their excessive emphasis on calculators. Most standards documents call upon students to use them starting in the elementary grades, often beginning with Kindergarten. Calculators enable students to do arithmetic quickly, without thinking about the numbers involved in a calculation. For this reason, using them in a high school science class, for example, is perfectly sensible. But for elementary students, the main goal of math education is to get them to think about numbers and to learn arithmetic. Calculators defeat that purpose. With proper restriction and guidance, calculators can play a positive role in school mathematics, but such direction is almost always missing in state standards documents.

Further more, it prevents practice of #2 below, thus degrading knowledge acquired earlier. Often it also prevents, in later grades, manual operations (division, multiplication, fractions operations, etc.), thus reducing practice of acquired knowledge, and thereby pushing those skills to a lower level.

2. Memorization of Basic Number Facts

Memorizing the “basic number facts,” i.e., the sums and products of single-digit numbers and the equivalent subtraction and division facts, frees up working memory to master the arithmetic algorithms and tackle math applications. Students who do not memorize the basic number facts will founder as more complex operations are required, and their progress will likely grind to a halt by the end of elementary school. There is no real mathematical fluency without memorization of the most basic facts. The many states that do not require such memorization of their students do them a disservice.

It is not just the “working memory” that is reduced. The ability to “see” the path to solving a problem is the part that suffers the most since the student cannot see beyond “what is 31×15″ [or anything of sort] and therefore cannot abstract to the overall path from the collection of calculations and operations required to walk that path.

However, be careful not to stress it too much. Each child would have his/her level of ability to do those things quickly. If your child can answer basic facts (such as multiplication of numbers up to 10, and addition of two digits numbers) reasonably quickly, i.e. without actual calculation, then s/he is good enough. Knowing those type of facts does not bring understanding of Math, but prevents learning difficulties.

3. The Standard Algorithms

Only a minority of states explicitly require knowledge of the standard algorithms of arithmetic for addition, subtraction, multiplication, and division. Many states identify no methods for arithmetic, or, worse, ask students to invent their own algorithms or rely on ad hoc methods. The standard algorithms are powerful theorems and they are standard for a good reason: They are guaranteed to work for all problems of the type for which they were designed. Knowing the standard algorithms, in the sense of being able to use them and understanding how and why they work, is the most sophisticated mathematics that an elementary school student is likely to grasp, and it is a foundational skill.

It is good (and important) to let students come up with ideas BUT, the standard algorithm should be then taught. Additionally, a good discussion requires a good teacher, one who KNOWS the underlying principles and can lead a Socratic method based discussion (i.e. guiding questions). It is also important for the teacher to recognize that most students will not have much to say and will not be able to come up with ideas and allow for that. There is a reason why it took thousands of years to develop our basic mathematical knowledge.

4. Fraction Development

In general, too little attention is paid to the coherent development of fractions in the late elementary and early middle grades, and there is not enough emphasis on paper-and-pencil calculations. A related topic at the high school level that deserves much more attention is the arithmetic of rational functions. This is crucial for students planning university studies in math, science, or engineering-related majors. Many state standards would also benefit from greater emphasis on completing the square of quadratic polynomials, including a derivation of the quadratic formula, and applications to graphs of conic sections.

This is why in my topics map Fractions is an important stepping stone. As I show in the topics map, ratio, proportions, percents, and probability [as well as Decimal fractions], are but extensions of Fractions. I guess there is a reason why my book on it is 110 pages long…;)

5. Patterns

The attention given to patterns in state standards verges on the obsessive. In a typical document, students are asked, across many grade levels, to create, identify, examine, describe, extend, and find “the rule” for repeating, growing, and shrinking patterns, where the patterns may be found in numbers, shapes, tables, and graphs. We are not arguing for elimination of all standards calling upon students to recognize patterns. But the attention given to patterns is far out of balance with the actual importance of patterns in K-12 mathematics.

Pattern recognition is a good ability to have. Practice should be made about it early on. However, it should be home based mostly. It does not contribute much beyond the early grades. It is also an innate ability (like music) that some would be naturally better at and some not as good. However, one can be absolutely daft at pattern recognition and still be an excellent mathematician [not just good at school Math]. There is a relationship between the two but it is not a necessary condition.

6. Manipulatives

Manipulatives are physical objects intended to serve as teaching aids. They can be helpful in introducing new concepts for elementary pupils, but too much use of them runs the risk that students will focus on the manipulatives more than the math, and even come to depend on them. In the higher grades, manipulatives can undermine important educational goals. Yet many state standards recommend and even require the use of a dizzying array of manipulatives in counterproductive ways.

I have been saying exactly that quite strongly to parents, and I am glad there is support! Manipulative, while helpful in understanding, can als hinder abstract thinking development if used too much, not just make the student depend on them for calculation.

7. Estimation

Fostering estimation skills in students is a commendable goal shared by all state standards documents. However, there is a tendency to overemphasize estimation at the expense of exact arithmetic calculations. For simple subtraction, the correct answer is the only reasonable answer. The notion of “reasonableness” might be addressed in the first and second grades in connection with measurement, but not in connection with arithmetic of small whole numbers. Care should be taken not to substitute estimation for exact calculations.

As I said on some boards, estimation is a good skill to have in life BUT it is completely unnecessary for understanding of Mathematics at any level. It is also one of those “innate skills” that can be taught to a certain level. The best way to get your kids to do estimation and see its value is exactly not in Math related work, where it has not value, but in life related issues, such as change, time, how much candy they get, how much work they have to do, etc.

8. Probability and Statistics

With few exceptions, state standards at all grade levels include strands devoted to probability and statistics. Such standards almost invariably begin by Kindergarten. Yet sound math standards delay the introduction of probability until middle school, then proceed quickly by building on students’ knowledge of fractions and ratios. Many states also include data collection standards that are excessive. Statistics and probability requirements often crowd out important topics in algebra and geometry. Students would be better off learning, for example, rational function arithmetic, or how to complete the square for a quadratic polynomial—topics frequently missing or abridged.

Probability, and specifically discrete probability that is taught in school, is actually simple and very difficult at the same time. The basic concept is simple (it’s a fraction, nothing more). Application to problems and understanding how it works can be difficult since it is not so intuitive to us. I have found it to be the most confounding aspect of Math with even college students, and even with my peers at the school of mathematics! You should only introduce it after Fractions, ratios, percents, and decimal fractions, have been mastered. You should do a lot of basic exercises as well. It can get tricky beyond the basic since one has to be a good “accountant” to do it right. BUT, there are some simple rules and ideas that make it much easier. Only they are not taught in most if not all books I’ve seen ☹ [and mine is not yet written…]

9. Mathematical Reasoning and Problem-Solving

Problem-solving is an indispensable part of learning mathematics and, ideally, straightforward practice problems should gradually give way to more difficult problems as students master more skills. Children should solve single-step word problems in the earliest grades and deal with increasingly more challenging, multi-step problems as they progress. Unfortunately, few states offer standards that guide the development of problem-solving in a useful way. Likewise, mathematical reasoning should be an integral part of the content at all grade levels. Too many states fail to develop important prerequisites before introducing advanced topics such as calculus. This degrades mathematics standards into what might be termed “math appreciation.”

The least studied topic, yet the most important. It is a subject/topic that is threaded through ALL of math since it is not subject specific [i.e. it’s not about Fractions, or about equations]. It should be studied as a subject all to itself with applications in specific subjects. See the discussion Denise and I have been having on the “How to solve word problems” blogs I wrote. If your child is good at fundamental, basic problem solving [i.e. regardless of subject matter], s/he is good at all problems. If not, s/he is good only at those subjects s/he mastered.

How to Read and Understand Word Problems?

July 7th, 2006


This is an excerpt from my forthcoming book: Solving Word Problem

What is this section about and how is it written?

Ø I explain the common wisdom about reading comprehension and how you should really “read and comprehend” in order to be able to translate the problem to Math.

Ø I write the common wisdom and cross it out to make it clear that you should not follow this common wisdom.

Ø I explain why it is the common wisdom.

Ø I then explain why the common wisdom is wrong.

Ø Then I explain what you should do and how to do it.

Ø Then you can read examples that show the ideas and make them less abstract.

Important note: the ideas you will read below about reading comprehension and translation are unorthodox, even radical (they go against what everyone is saying). Don’t believe me. Try them and you will see they work.

Ø If you are older then 3rd grade age, you might be already used to the common method of reading and translating (to math) word problems. It takes a while to get rid of those bad habits and be able to use the approach I show you. But it will become easier quickly and you should start seeing results quite early in the process. If not, contact me at help@learningbyyourself.com and I will guide you through it.


Common Wisdom #1

1. Read the whole problem at least once (possibly two or three times) and build a picture/story/image  of it.

Why is it the common wisdom?

It is the common wisdom because in order to understand a general paragraphs (such as in stories, or an article about something) you do have to understand the overall idea first in order to understand how the parts connect to it and to each other.

Why is it Wrong for Word Problems?

It is not completely wrong BUT most of the time it is:

Ø Not efficient. You can often solve the problem FASTER if you don’t read the whole problem but start translating as you go.

Ø Information overload (too much information to understand) and confusion. Word problems usually provide a lot of information. As a result, by the time you reach the end of the paragraph you have too much information to make sense of.

Ø Word problems are NOT a general paragraph or an article. Word problems are a description of a problem that has to be solved. Therefore, your objective is not to understand the story, but to solve the problem. In order to solve you need to translate the statement to math, and that you can do as you read instead of at the end after you read the whole problem.

o Understanding the story helps but does not guarantee success. Therefore, it is good if you do understand the story, but it is not necessary.

Q: how can I solve a problem without understanding all of it as a whole?

A: by using the reading comprehension, translation, and problem solving strategy I show you. With those, EVERY problem becomes a series of small steps instead of one big jump. It is easier to advance one small step at a time instead of big jumps. Interestingly, it is also faster this way.


Common Wisdom #2

2. Read each sentence, understand, and translate it, from the beginning to the end. You are told to read from the beginning to the end, each sentence, each paragraph, and the whole problem description. The same way you just read those sentences.

Why is it the common wisdom?

It is the common wisdom because this is how we write our thoughts.

Why is it Wrong for Word Problems?

Ø Reading and comprehending in the regular way does not necessarily work for word problems because you need to translate word problems to math.

Ø Math as a language is built from the inside out (or end to beginning).

Ø In many word problems, each sentence has several parts that depend on each other. To understand the sentence you have to understand how the parts are connected. However, if you don’t understand the last part of the sentence, it is difficult to understand how the parts before it connect to it.

Ø Therefore, you start translating from the “end” of the sentence (the last concept mentioned) and build the layers backwards to the beginning of the sentence.

o If the sentence has three concepts: the concept at the beginning of the sentence depends on the concept in the middle, which depends on the concept at the end of it.

Ø Fundamentally, it is the same issue as the common wisdom #1. Instead of trying to understand a collection of ideas together, you want to break them to their parts and understand each one separately. You want to start with the small idea first and gradually build bigger ideas (sentences).

Important note: simple word problems (grades k-3 or 4) often do not have those complications (they have simple sentences) and therefore the regular method works ok. The problem comes up later when students have gotten used to the “regular” method of reading comprehension but face more complicated descriptions and concepts.

The Math Wars: Asking the Wrong Questions

May 23rd, 2006

Having taught at two countries and at multiple levels (elementary to university) I have seen that Math is a universal problem. Many countries struggle with how to teach math, and even those that do well do not argue that it is easy to achieve. Therefore, the conclusion must be that it is not a cultural issue, as many in the US like to believe, although culture and values can lessen or strengthen the problem.

The Math Wars got it right, though neither camp is right. To distill the approaches, we can say that one camp, supported by education researchers, favors discovery mode, where kids discover solution approaches by themselves and learn to explore math, and at its strongest approach relegating algorithms (solution methods) non-existent. The other camp, supported by mathematicians, and sometimes confused with “traditional math”, favors a strong technical foundation. While the first camp has been winning on the curriculum front and most school district in the US are teaching variants of the “New Math” (Discovery Math, etc.), the US test scores on the international test (TIMSS – see www.timss.org) have fell, and as students grow they do worse on the test, lending support to the “algorithmic” camp. The arguments on each side are complex at times and have devolved into what the title implies: a war. It is no longer a discussion but more of a shouting match.

As a mathematician I KNOW that the discovery type of curriculum is bad for kids but at the same time, as a long time teacher, I KNOW that rote learning of algorithms is not enough either. The problem is that each side is answering a different question, and both are the wrong questions!

The “discover/new math” camp asks, “How do kids learn and how can we facilitate such learning?” It is a question that is natural for education researcher but it is the wrong question since ALL of us have flaws in the way we learn. While some of us are better at it than others, without guidance most if not all would end up with some faulty problem solving approaches. As millennia of experience show, learning how to think SYSTMATICALLY requires long and rigorous training. As I have constantly seen with my college students, in some of the top universities in the US, the education system fails in teaching them how to systematically analyze, since it is not taught AT ALL. We can think of the question posed using my favorite metaphor of sports. A coach would not ask, “How do kids shoot and how can we facilitate such?” because the coach knows most kids do not naturally end up with the right method of shooting. While it is a critical and important question to ask, it is not the RIGHT and main question, but rather a supporting question.

The “algorithmic” camp asks, “How do we make sure kids know how to solve the math problems they study in school and college?” While in terms of ensuring performance in Math it is a better question than the “discovery” one, it is also the wrong question. As studies across nations have shown, technical skills are important for understanding math concepts. But, technical skills, while they might ensure good scores on standardized tests, do not ensure deep understanding of concepts. However, it is the wrong question because it focuses on Math skills, and not thinking and learning. Having seen thousands of college students who mostly got high grades in their studies, Math included, I have learned that their general analysis capabilities and skills, as well as understanding of basic mathematical concepts, are severally lacking at best.

To reach the right question we have to examine what the objectives of education (the “thinking” part of it), and not just Math education are, or should be. There are numerous important objectives that have to do with how students approach learning and specific topics (sense of wonder and joy, curiosity, etc.) and even broader ones that has to do with their social and personal life. But to distill the issue at the center of the Math Wars I focus on the intellectual/mental elements only.

There are three elements that education should strive to achieve in students:

  1. Ability to learn.
  2. Systematic analysis, thinking, and problem solving abilities.
  3. Domain specific knowledge and skills.
  4. The ability to apply the second to the third.

The first objective is general to all topics and subjects and provides students with the fundamental capabilities they need to approach any domain of knowledge, be it Math or Literature. The second is obvious and arrives from the need to master certain knowledge and skills, although which ones can be argued on. The third is but a merger of the first two, but it is not necessarily trivial.

It is clear that the “discovery” camp tries to answer a question related to the first objective, believing that answering the “how kids learn?” question leads to achieving that first objective, which in turn results in achieving objectives 2-4, when used in a specific domain, such as Math. The problem is that facilitating students learning does not necessarily (and that word is critical) results in students achieving even #1. They may, or they might not learn how to learn. Further more, even if #1 is achieved, #2, #3, and #4 might not be, since the link is not automatic.

It is also clear that the “algorithmic” camp focuses on the third objective. As a result, while this approach would result in higher Math capabilities, it would not guide students to develop to their full potential, and might kill their internal curiosity and sense of joy in learning, due to over emphasis on rote and drills. It would also not teach them how to learn, or systematically analyze and solve problem, not just in Math.

It is important to note that when students know how to learn, and know how to approach problems, they enjoy the specific domain in which they have those capabilities. This is why those who are good at Math tend to enjoy it, and why those who are good at sports tend to enjoy sports. Naturally, enjoying an activity creates a strong incentive to learn and practice it, thereby creating a positive cycle. But it is the ability that drives joy, or can kill it if absent.

The right question is therefore simple:

How do we achieve all four objectives?

I will discuss and answer it in my next Blog ☺