A boy is 6 years younger than his sister. If he is (x-9) years old, how long will it take for his si

A boy is 6 years younger than his sister. If he is (x-9) years old, how long will it take for his sister to be x years old?

If the boy is x - 9 years old, and he's 6 years younger than his sister, than the sister is older by 6 years.
Sister's Age = x - 9 + 6
Sister's Age = x - 3

In order to be x years old, we must add 3 years:
x - 3 + 3 = x

So in 3 years, the sister will be x years old.