When Tom was 2 years old his sister was half his age. Now, Tom is 100 years old. Then how old is his sister?
Answer
His sister is 99 years old. When Tom was 2 years old, his sister was 1 year old. 98 years later, he becomes 100 years old. And 98 years later, his sister will be 99 (98 + 1) years old.