A girl is 18 years old and her brother is a third of her age. When the girl is 36 what would be the age of the brother?

First we need to calculate how many years her brother has when she is 18. As text says he is:

18/3 = 6 years old

Now when she gains another 18 years (36-18=18) he will also gain those 18 years

When she is 36 he will have 6+18=24 years old

Answer is 24.

Her brother is 6 yrs old