Age calculation

Rohit was 4 times as old as his son 8 years ago. After 8 years, Rohit will be twice as old as his son. What are their present ages?

Answer: 
Son's present age = 16 years
Rohit's present age = 40 years


Explanation:
Let son's age 8 years ago be x years. Then, Rohit's age 8 years ago = 4x years.
Son's age after 8 years = (x+8)+8 = (x+16) years.
Rohit's age after 8 years = (4x+8)+8 = (4x+16) years.
2(x+16) = (4x+16)  <=> 2x = 16 <=> x = 8.
Hence, son's present age = (x+8) = 16 years.
           Rohit's present age = (4x+8) = 40 years.