8 years ago , the average age of husband, wife and child was 42 years old and 9 years ago the average age of husband and wife was 52 years. What is present age of child ?

28

8 years ago total age of ( H + W + C ) = 42 × 3 = 126

At present total age of ( H + W + C ) = 126 + (8 × 3) = 150

9 years ago total age of ( H + W) = 52 × 2 = 104

At present total age of (H + W) = 104 + (9 × 2) = 122

Present age of Child = 150 - 122 = 28