Five years ago the average age of husband and wife was 23 years. At present the average age of husband, wife and child is 20 years. The present age of child is?

4 years

Average age of husband and wife = 23 years (5 years ago)

Average age of husband and wife = 23 + 5 = 28 years (at present)

Sum of present age of husband and wife = 28 × 2 = 56 years ....(i)

Average age of husband, wife & child = 20 years (at present)

Sum of present age of husband, wife and child = 20 × 2 = 60 years ....(ii)

(ii) - (i)

Present age of child = 60 - 56 = 4 years.