The hard kind of percent problem is this one:
John paid $52.50 for a shirt including tax of 5%.
What was the price of the shirt before tax?
I imagine this problem is a cinch (cinch?? sp?) for the gifted, but for the non-gifted, this problem is HARD.
I was thinking this problem is an example of a
partitive word problem, but now, re-reading Carolyn's original post on the subject, maybe not.
variations on a theme: 3 kinds of percent problemsAs far as I can tell, every percent problem comes in three forms. (PLEASE correct me if I'm wrong.)
price of shirt: $50
tax: 5%
price of shirt including tax: $52.50
1. A shirt sells for $50. Sales tax is 5%. What will the total cost be?
2. A shirt sells for $50. After tax, the shirt sells for $52.50. What is the tax rate?
3. A shirt sells for $52.50 including a tax of 5%. What was the original price of the shirt?
Problem number 1 is easy.
Making the move to problems 2 and 3 is hard for many students.
But 3 is the killer. (I think.)
If you're having to teach or reteach percent to your children, be sure to include lots of number 3s in the mix.
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Do math textbooks call numbers 2 and 3 "
work backwards problems" problems these days?
They may.
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Liping Ma deals with these problems in her chapter on division by fractions.
She describes asking her group of U.S. elementary teachers to create a word problem representing 1 3/4 ÷ 1/2:
Imagine that you are teaching division with fractions. To make this meaningful for kids, something that many teachers try to do is relate mathematics to other things. Sometimes they try to come up with real-world situations or story-problems to show the application of some particular piece of content. What would you say would be a good story or model for 1 3/4 ÷ 1/2?
She goes on to say:
...division by fractions is an advanced topic in arithmetic. Division is the most complicated of the four operations. Fractions are often considered the most complex numbers in elementary school mathematics. Division by fractions, the most complicated operation with the most complex numbers, can be considered as a topic at the summit of arithmetic.
The summit of arithmetic!
I love it!
The U.S. teachers didn't fare well.
Of the 23 U.S. teachers, 212 tried to calculate 1 3/4 ÷ 1/2. Only nine (43%) completed their computations and reached the correct ansewr. For example, Mr. Felix, a beginning teacher, gave this explanation.:
I would convert the 1 3/4 to fourths, which would give me 7/4. Then to divide by 12/2, I would invert 1/2 and multiply. So, I would multiply 7/4 by 2 and I would get 14/4, and then I would divide 14 by 4 to get it back to my mixed number, 3 2/4 or then I would reduce that into 3 1/2.
[snip]
Tr. Bernadette, the experienced teacher who was very articulate about the rationale for subtraction with regrouping, tried a completely incorrect strategy:
I would try to find, oh goodness, the lowest common denominator. I think I would change them both. Lowest common denominator, I think that is what it is called. I do not know how I am going to get the answer. Whoop. Sorry.
Tr. Bernadette sounds a little like
me trying to do a simple percent change problem the other night. C. and I looked at the same problem last night and we
both said: how did we make this so complicated?
That is the $40,000 dollar question.
The $40,000 dollar answer is:
failure to transfer.
Ma goes on to discuss the word problems U.S. teachers came up with to represent 1 3/4 ÷ 1/2, and there we see a clean sweep: only one of the 23 teachers produced a correct model of the problem, and that one model was "pedagogically problematic."
I am proud to say that when I read Ma I was immediately able to produce a mathematically correct word problem representing 1 3/4 ÷ 1/2. As I recall my problem went something like this:
Catherine has two dogs, and each dog eats 1/2 can of dog food every morning. If she has 1 3/4 cans of dog food left, how many servings is this?
Six of the U.S. teachers confused dividing by 1/2 with dividing by 2.
The Chinese teachers created two genres of problems:
- quotitive division, which Ma calls the measurement model
- partitive division - finding a number such that 1/2 of it is 1 3/4
Which brings me back to my hard percent problem.
At least, I think it does.
the "measurement" modelThe measurement model is more obvious to me. Obvious meaning easy to understand. My own word problem falls in the measurement category: how many 1/2s are in 1 3/4?
How many 1/2s can 1 3/4 be divided into?
How many 1/2-can servings of dog food are there in 1 3/4 cans of dog food?
If I'm measuring the number of 1/2s in 1 3/4, I divide 1 3/4 by 1/2.
1 2/3 ÷ 1/2 = 3 1/2.
I have 3 1/2 servings of dog food.
the "partitive" modelChinese teachers gave the measurement model short shrift:
Among more than 80 story problems representing the meaning of 1 3/4 ÷ 1/2, 62 stories represented the partitive model of division by fractions--"finding a number such that 1/2 of it is 1 3/4":
Division is the inverse of multiplication. Multiplying by a fraction means that we know a number that represents a whole and want to find a number that represents a certain fraction of that.
After all this time, I still find this passage mystifying.
I was even more mystified when I read the story problem Tr. S came up with:
My story will be: A train goes back and forth between two stations. From Station A to Station B is uphill and from Station B back to Station A is downhill. The train takes 1 3/4 hours going from Station B to Station A. It is only 1/2 time of that from Station A to Station B. How long does the train take going from Station A to Station B?
[isn't this written wrong? shouldn't the numbers be reversed?]
I'm going to reverse them:
My story will be: A train goes back and forth between two stations. From Station A to Station B is uphill and from Station B back to Station A is downhill. The train takes 1 3/4 hours going from Station A to Station B. It is only 1/2 time of that from Station A to Station B. How long does the train take going from Station B to Station A?
update: see belowHere's another:
The mom bought a box of candy. She gave 1/2 of it which weighed 1 3/4 kg to the grandma. How much did the box of the candy originally weight? (Ms. M.)
For a lot of us, the candy problem will be most obviously similar to the "hard" percent problem.
To solve the candy problem:
let
x = original weight
1/2
x = 1 3/4
1 3/4 ÷ 1/2 =
x3 1/2 =
xThe original box weight 3 1/2 pounds.
The hard percent problem has the same form. We know the final price; we know the tax rate. We need to know the price of the shirt
sans tax.
let
x = price of shirt
1.05
x = 52.50
52.50 ÷ 1.05 =
x50 =
xprice of shirt = $50
I have to include this for people like me: the reason you take 1.05x is that it is a shortcut.
I spent years of my life taking 5% of $50 to get the sales tax ($2.50), then adding the sales tax amount to the price of the shirt.
Then C's 5th grade teacher explained to me that instead of doing these two steps I could simply multiply the price of the shirt by 1.05 and get the whole thing over with.
The "1" in the 1.05 ensures that the price of the shirt is part of the final value.
That was a revelation.
are partitive problems harder than measurement problems?I still don't know what kind of language to use in describing these problems (if I'm not going to use the term "partitive," that is, and for the time being I am not).
Can we say that the shirt problem is a part - part - whole problem in which the whole and a part are known and we have to find the other part?
hmmmm.....
Not exactly.
unknown part: price of shirt
unknown part: sales tax in dollars
part: sales tax rate
whole: final price
I have no idea what to call these problems or even how to describe them.
All I know is that they are difficult for kids to learn, and they don't "come naturally" once a student has learned how to find the price of a shirt given the original price and the sales tax rate.
Any thoughts?
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update (from above)I'm thinking there is a translation problem with the train problem as printed in Ma's book:
My story will be: A train goes back and forth between two stations. From Station A to Station B is uphill and from Station B back to Station A is downhill. The train takes 1 3/4 hours going from Station B to Station A. It is only 1/2 time of that from Station A to Station B. How long does the train take going from Station A to Station B?
As it stands, this problem does not model 1 3/4 ÷ 1/2.
This problem models 1 3/4 x 1/2.
I think the problem should read:
My story will be: A train goes back and forth between two stations. From Station A to Station B is uphill and from Station B back to Station A is downhill. The train takes 1 3/4 hours going from Station B to Station A. It is only 1/2 time of [the time it takes to go from] Station A to Station B. How long does the train take going from Station A to Station B?
let
x = time it takes to go from Station A to Station B
time it takes to go from station B to Station A: 1 3/4 hours
1/2
x = 1 3/4
x = 1 3/4 ÷ 1/2
x = 3 1/2
It takes 3 1/2 hours to go from Station A to Station B