Aboy is 5 years old and his sister is 3 times as old as he is when the boy is 10, how old will his siter be

When the boy is  boy is 5 years old, his sister is 3 times as old as he. This means his sister is 15 years old at that time. we can notice in the ages that there is an age gap of 10 years. Thus, when the boy is 10, his sister is 10 + 10 or equal to 20 years old.