SOLUTION: Suzy is 5 years younger than her brother Joey. The sum of their ages is 25. How old is Suzy?

Solve that system by 

1. substituting (J-5) for S in the second equation,

2. Solve for J, which will by Joey' age.

3. Substitute the number you get for J in S = J - 5
and simplify.  That will be Suzy's age.


Edwin

Let Suzy's age be S

Then Joey's is: S + 5

We then get: S + S + 5 = 25

2S = 20

Suzy's age, or