P. 4003. On a running training the runners run at a speed of v=5 m/s along a straight path, forming an L=35 m long column. Their coach runs towards them at a speed of u=2 m/s. The first runner turns back and runs in the same direction as the coach, still at a speed of v, when he meets the coach, and so do all the runners when they meet their coach.
a) How much time elapses between the turn of the first and the last runner?
b) How long is the column when they all run in the same direction as the coach?