The sum of the present ages of a father and his son is 57 years. Six years ago, father's age was four times the age of the son. After 6 years, son's age will be:  Choose the correct choice from the following answers
A) 17 B) 18 C) 19 D) 20 E) 21
Let the present ages of son and father be x and (57 - x) years respectively.   Then, (57 - x) - 6 = 4(x - 6)   51 - x = 4x - 24   x = 15   The son's age after 6 years will be 21.   The answer is 21.
<result>E</result>
The sum of the present ages of a father and his son is 57 years. Six years ago, father's age was four times the age of the son. After 6 years, son's age will be: