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
```

What did I do wrong in using just one variable?