It's a complicated algebra problem. She does pretty good considering that she doesn't have pencil, paper, and a calculator.
The big brother is 4 years old. His little brother is half his age. When the big brother is 100 years old, how old will the little brother be?
-via Tastefully Offensive
I can't figure out why, but first instinct seems to be to take 1/2 of 100 and say that's the age of the younger brother. That's not the case. The younger brother is only 2 years younger than the older. Always.