Five years ago the average of husband and wife was 23 years. Today the average age of husband, wife and child is 20 years. How old is the child?

4 years

5 years ago, 

Average age of husband and wife is = 23 years 

Sum of ages of husband and wife = 23 × 2 = 46years

 Sum of  present ages of husband and wife = 46 + 10 = 56 years

At present , the average age of husband , wife and child = 20 years 

Sum of  present ages of husband, wife and child = 20×3 = 60 years 

Age of child= 60 - 56 = 4 years