5 A stone is projected from a point on horizontal ground with a speed of \(20 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) at an angle of \(\alpha ^ { \circ }\) above the horizontal. The stone is moving horizontally when it hits a vertical wall at a point 7.2 m above the ground.

1. Find the value of \(\alpha\). After rebounding at right angles from the wall the speed of the stone is halved. Find
2. the distance from the wall of the point at which the stone hits the ground,
3. the angle which the direction of motion of the stone makes with the horizontal, immediately before the stone hits the ground.