Question

Two trolleys of mass m each, coupled together and moving with a certain
velocity v, collide with an exactly similar pair of stationary trolleys.
Considering the collision to be elastic, use the law of conservation of
momentum to find the velocity of the combination of all the trolleys after
collision.

Answer 1

Explanation:

2m × v + 2m × 0 =( 2m+ 2m) v¹

2mv = 4m v¹

v ¹ = v/2

but as you asked velocity of combination of trolleys the collision can't be elastic

It is perfectly inelastic collision