In the figure, the block ‘B’ of mass ‘m’ starts from rest at the top of a wedge ‘W’ of mass ‘M’. All surfaces are without friction ‘W’ can slide on the ground ‘B’ slides down on to the ground, moves along it with a speed ‘V’ has an elastic collision with the wall, and climbs back on to ‘W’

From the beginning, till the collision with the wall, the centre of mass of ‘B’ plus ‘W’ does not move horizontally

After the collision, the centre of mass of ‘B’ plus ‘W’ moves with the velocity  $\frac{2mV}{m+M}$

When ‘B’ reaches its highest position of W, the speed of W is  $\frac{2mV}{m+M}$

When B reaches its highest position of W, the speed of W is   $\frac{mV}{m+M}$