Purple math age word problem example  TOPIC_SOLVED

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Purple math age word problem example  TOPIC_SOLVED

Postby upspin on Tue Sep 15, 2009 2:02 am

Hello.

My question regards the first age example problem on this page (http://www.purplemath.com/modules/ageprobs.htm).

In January of the year 2000, I was thirteen times as old as my son William. In January of 2009, I was four times as old as him. How old was my son in January of 2000?


The author uses 2 variables to solve this problem however, can it be solved using one variable?

I tried and my answer did not match the answer when using 2 variables to solve the problem.

Here's what I did when using x as William's age.

Code: Select all
              Age now                 Age in 9 years
Elizabeth         13x                        4(x+9)
William            x                         (x+9)


Setting up my equation, I get:   

13x = 4(x+9)
13x = 4x + 36
9x = 36
x = 4


So, William's age was 4 in January of 2000. But the author uses 2 variables for the problem and comes up with the age of 3.

What did I do wrong in using just one variable?
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Postby stapel_eliz on Tue Sep 15, 2009 1:12 pm

upspin wrote:...can it be solved using one variable?

Certainly! Instead of using variables for each of the original ages, define the other age in terms of the variable for William's age! :D

upspin wrote:So, William's age was 4 in January of 2000. But the author uses 2 variables for the problem and comes up with the age of 3.

You did nothing wrong. The "author" was an idiot! :oops: :lol:
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