The average age of a family of 5 members is 20 years. The age of the youngest member is 4 years. What was the average age of the family at the birth of the youngest member?

Explanation

Sum of ages now = 5 * 20 = 100 years. 4 years ago, sum of ages of 5 members = 100 – 5(4) = 80 years. Average age at birth = 80 / 4 = 20 years… Wait, if 5 members, 4 years ago sum is 80, average of 4 members or 5 members? Usually divided by 4 or 5. Divided by 4 = 20 years.