Word Problem with Percents - an example
November 12th, 2006Below 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.”
** **
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.