Brain Teasers Brain Teaser – March 18, 2017 March 18, 2017 Share on Facebook Tweet on Twitter A grandfather is 56 years old, and his grandson is 14 years old. After how many years will the grandfather be twice as old as his grandson? See the answer Answer: After 28 yeas. Let X be the number of years we are looking for. Then, 56+X=2(X+14). Solving for X, we get X=28.