The sum of the two girls' ages is 24, and the difference between their ages is 6. What are the girls' ages?

First, let's create two variables to express the girls' ages.
x=age of girl 1
y = age of girl 2
Now, let's create our expressions
x+y = 24
x-y=6
Let's solve by elimination
2x = 30
Divide both sides by 2
x=15
We can plug 15 in for x into one of our expressions
15 + y = 24
Subtract both sides by 15
y = 9
The older girl is 15 years old and the younger girl is 9 years old.