Question 1084817
It will never happen.  Rajesh is younger than Reeta.   
===============
If you meant  'Rajesh is now 20 years old and reeta is 4 years older than him.In how many years Rajesh be twice as old as Reeta IS NOW?'     Then the answer is 28 years b/c you're looking for when Rajesh is 48 years old (2x Reeta's CURRENT age).   

Rajesh's age = 20
Reeta's age = 20+4 = 24
x = number of years until Rajesh is twice Reeta's current age

  20+x = 2(24)     <<< this is the formulation, or equation that needs solving
  20+x = 48          
        x = 48 - 20   
        x = 28           <<< answer is found by solving the original equation for x