Use the law of conservation of momentum to solve this problem. We have a system of two bodies (bullet and block). Initially, only the bullet has a non-zero momentum. After the collision, both have some momentum and we know the part for the bullet, so it is simple to isolate the part for the block. Call v_t0 the initial bullet speed, v_t1 new bullet speed, v_k speed of block. (similarly for masses):
[tex]m_t\cdot v_{t0}=m_t\cdot v_{t1} + m_k\cdot v_k\implies\\v_k=\frac{m_t\cdot v_{t0}-m_t\cdot v_{t1}}{m_k}=\frac{0.01kg\cdot(400-300)m/s}{0.5kg}=2\frac{m}{s}[/tex]
The block will move with a speed of 2 m/s in the direction of the bullet.
