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?

Her brother is 6 yrs old

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.