Question

Two identical balls each of mass 4kg are moving towards each other with speeds 2m/s and 3m/s respectively. They undergo head on perfectly elastic collision. Then impulse imparted by one ball on other is

Answer 1

An Elastic collision between balls of equal mass swap velocities.  
So impulse = mΔv = 4kg * (3m/s - -2m/s) = 20 kg·m/s = 20 N·s 

Answer 2

Two balls of equal mass can be undergoing head on collision and thus it determines moving part that is relatively goes velocity of separation.

In general, An Elastic collision between balls of equal mass swaps velocities.  

So impulse = mΔv = 4kg * (3m/s - -2m/s) = 20 kg·m/s = 20 N·s.