BITSAT 2012 Paper

Momentum of the man + platform is a constant and is 0.
Reason is that there is no net external force acting on this system.
Hence as per Newton's 2nd law, the momentum is conserved.
We take the velocities wrt to ice (static/inertial reference).
Let velocity of the recoil of the platform be V m/s.
Then the man is moving wrt the ice in the opposite direction at (30-V) velocity.
100 × (30-V) - 200 × V = 0
300 V = 3000
v = 10
So Recoil velocity V = 10 m/s in the opposite direction of movement of the man.