A father is now three times as old as his son. Nine years back, he was four times as old as his son. The age of the son (in years) is  Choose the correct choice from the following.
A) A)12
B) B)15
C) C)18
D) D)27
E) E)22
Explanation:   Let son's age be x years. Then, father's age = (3x) years.   Nine years ago, father's age = (3x - 9) years and son's age = (x - 9) years.   So, 3x - 9 = 4 (x - 9)=&gt; 3x - 9 = 4x - 36=&gt; x = 27.   Answer: 27
<result>D</result>
A father is now three times as old as his son. Nine years back, he was four times as old as his son. The age of the son (in years) is