The average age of husband, wife and their child 4 years ago was 25 years and that of wife and the child 6 years ago was 18 years. The present age of the husband is:

39 years 

4 years ago, The average of the husband, the wife and the child was 25 years .

Sum of the age of husband , wife and child is  = 25 × 3 = 75

Sum of the present age of husband , wife and child is = 25 × 3  + 12 = 87  years

6 years ago,  The average of the wife and the child was 18 years.

Sum of the age of  wife and child is  = 18 × 2 = 36

 Sum of the ages of wife and child = 18 × 2  + 12 = 48 years 

The husband’s age, now =  87 –48  =  39 years.