Physics Problems: Conservation of Momentum

I'm completely stumped on a few problems that are due after this weekend. If anyone could offer me hints on how to start them, it would be great. Thanks!

1. A 50 g bullet lodges into a 2.0 kg block of clay hung by a string. The bullet and clay rise together to a height of 12 cm. What was the velocity of the 50 g mass just before entering?

2. A 0.150 kg bullet is fired at 715 m/s into a 2.0 kg wooden block at rest. The velocity of the block afterward is 40 m/s. The bullet passes through the block and emerges with what velocity?

3. A 2.0 kg ball moving to the right at 1.0 m/s strikes a 4.0 kg ball moving left at 3.0 m/s. What are the velocities after impact, assuming complete elasticity?

4. A 2.0 kg block A and a 1.0 kg block B are pushed together against a spring and tied with a cord. When the cord breaks, the 1.0 kg block moves to the right at 8.0 m/s. What is the velocity of the 2.0 kg block?

**Edit:** It would appear the OP now only requires help with Questions 1. and 3.