According to the law of conservation of momentum, initial momentum = final momentum.

Momentum is the product of mass and velocity.

Initial momentum = m.v = 50x200 = 10000 g.m/s

Final momentum should also be 10000 g.m/s

Now, mass = 50+950 = 1000g (since bullet is stuck in wood block).

So velocity = Final momentum/ mass

= 10000/1000 = 10m/s