Last week’s answer: Diane is 20 years old. The average before Diane is (9 + 12 + 15) / 3 = 12 years old. Therefore the average after her is 12 + 2 = 14. So the total of all of their ages is 14 x 4 = 56. Subtract the original total of ages (9 + 12 + 15 = 36) to get 56 – 36 = 20.
This week’s problem: What is the first square number greater than 400?
The answer will be in next week’s problem.