The ages of the four members of a household add up to 124 years today. How many years from now will their ages add up to 168 years?

The total has to climb by 168 - 124 = 44 years. Every year that passes adds 1 year to each of the 4 members, so the total climbs by 4 years annually, and 44/4 = 11 years. (Check: in 11 years the four have gained 44 years and 124 + 44 = 168.) 44 is the rise in the total, mistaken for a wait in years. 42 is 168/4, the average age at that future date, and 31 is 124/4, the average today - both are ages, not spans of time.