SOLUTION: A girl is 21 years old and her brother is a third of her age. When the girl is 36, what will be the age of her brother?

The girl (the young women ?) is 21 years old and the brother is 7 years old, according to the condition.

Hence, the girl is 14 years older than her brother, and this difference remains unchangeable forever while they are alive 
(at least, in the age word problems it is so). 


Therefore, when the women will be 36 years old, her brother will be 36 - 14 = 22 years old.