The average age of 7 members of a family is 24 years. If the youngest member is 4 years old, then what was the average age (in years) of the family including the youngest member at the time of the birth of the youngest member?

Currently, the total age of 7 members is 7 × 24 = 168. The youngest is 4, so the other 6 members sum to 168 - 4 = 164. Four years ago (at the youngest's birth), these 6 members were each 4 years younger, so their sum was 164 - 24 = 140. Including the newborn (age 0), the total was 140 for 7 members, giving an average of 140/7 = 20 years.