The average age of 4 member of family is 20 yr. If youngest member is 4 yr. old, What was the average age of family at the time of the birth of youngest member?

Total age of all members of the family (at present) = 20 x 4 = 80
4yr ago, total age of all members = 80 - ( 4 x 4) = 80- 16 = 64 yr.
As 4 yr ago, there were only 3 members in the family
∴ Average age of the family at the time of the birth of the youngest member
= 64/3
= 21¹/₃ yr