20 years ago, the average age of a husband and his wife was 23 years. Now, the average age of the family consisting of the husband, wife and their son is 34 years. The present age of the son is:

Correct option is B

Given:

20 years ago, the average age of the husband and wife was 23 years.

The current average age of the family (husband, wife, and son) is 34 years.

Solution:
Let the present age of the husband be h, the present age of the wife be w, and the present age of the son be s.

The average age of the husband and wife 20 years ago is given as 23 years.

​$$\frac{h - 20 + w - 20}{2} = 23$$

h + w - 40 = 46

​$$h + w = 86 \quad \text{(Equation 1)}$$

The current average age of the family (husband, wife, and son) is 34 years:

​$$\frac{h + w + s}{3} = 34$$​

​​​$$h + w + s = 102 \quad \text{(Equation 2)}$$​​

Subtract Equation 1 from Equation 2.

(h + w + s) - (h + w) = 102 - 86

s = 16
The present age of the son is 16 years.