Consider the question. John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be?
 If the answer is '60'; explain the reasoning:
His sister is 10 * 2 = 20 years old. Their age difference is 20 - 10 = 10 years. Therefore when he is 50 she will be 50 + 10 = 60 years old.