Let x be the number of years to pass. Then your basic equation is this 40 + x = 2*(10+x) Simplify and find x 40 + x = 20 + 2x 40 - 20 = 2x - x 20 = x ANSWER. In 20 years. CHECK. In 20 years their ages will be 10+20 = 30 (the son) and 40 + 20 = 60 (the father).