Momentum, K.E. Relation

Q. A bullet of mass $20\text{g}$ and moving with $600\text{m/s}$ collides with a block of mass $4\text{kg}$ hanging with the string. What is velocity of bullet when it comes out of block, if block rises to height $0.2\text{m}$ after collision?
(Take $g=10{\text{m/s}}^2$)

1. $200\text{m/s}$
2. $150\text{m/s}$
3. $400\text{m/s}$
4. $300\text{m/s}$

Q. Two ice skaters A and B approach each other at right angles. Skater A has a mass $30\text{kg}$ and velocity $1\text{m/s}$ skater B has a mass $20\text{kg}$ and velocity $2\text{m/s}$. They meet and cling together. The final velocity of the couple is

1. $2\text{m/s}$
2. $1.5\text{m/s}$
3. $1\text{m/s}$
4. $1.5\text{m/s}$

Q. A particle of mass $1\text{kg}$ suffers one - dimensional elastic collision with an unknown mass at rest. It rebounds directly backward by losing $64\%$ of its initial kinetic energy. The unknown mass $\left(\text{in kg}\right)$ is