John is 10 years old. His sister is twice his age. When he is 50 years old, how old will his sister be? Think carefully first, then make a decision:
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. So, the answer is 60.