The sum of the present ages of a father and his son is 42 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:
A) 14 B) 16 C) 18 D) 20 E) 22
Let the present ages of son and father be x and (42 - x) years respectively.   Then, (42 - x) - 6 = 4(x - 6)   36 - x = 4x - 24   x = 12   The son's age after 6 years will be 18.   The answer is 18.
<result>C</result>
The sum of the present ages of a father and his son is 42 years. Six years ago, father's age was four times the age of the son. After 6 years, son's age will be: