Consider the question. Eustace is twice as old as Milford. In 3 years, he will be 39. How old will Milford be?
If the answer is '21'; explain the reasoning:
Eustace's current age must be 39 years old 3 years = 36 years old. So Milford's current age must be 36 years old / 2 = 18 years old. So in 3 years, Milford will be 18 years old + 3 years = 21 years old.