7. Two billiard balls of equal mass, one blue and the other red, slide towards each other at 2.0m/s in opposite directions towards each other. They collide head on and rebound both at 1.8m/s in opposite to their original
velocities, a s shown in the diagram below. Show how momentum is conserved in this collision.

P = M V    expresses momentum

Both before and after collision the momenta are equal and in opposite directions. So the the total momentum is zero both before and after the collision. P is a vector quantity as is the velocity. Total momentum must be zero.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.