12. A hiker hears an echo 1.5 s after shouting over a valley. Estimate the width of the valley. 6. Two billard balls of equal mass undergo a perfeclty elastic head-on collison. If the speed of the white ball was initaially 2.00 m/s, and of the 8-ball 3.00 m/s in the oposite direction, what will be the final speed of the 8-ball? 7. A ball of the mass 2.0 kg moving east with a speed of 4.0 m/s collides head-on with a 0.92 kg ball at rest. If the collision is perfectly elastic, what will be the speed and direction of each ball after the collision?