A boy is four years old and his sister is three times as old as he is. When the boy is 12 years old how old will his sister be?

If the boy is four years old and his sister is three times as old, then she is 12 years old. The age difference between them is 8 years (12 - 4). When the boy is 12 years old, his sister will be 12 + 8, which makes her 20 years old.