The angles made by the windscreens of two motorcars with horizontal are β1=30º  and β2=15º  respectively. The first car is travelling with a velocity v1  horizontally. The second car is travelling with a velocity v2  in the same direction. The hail stones are falling vertically. Both the drivers observe that the hail stones rebound vertically after elastic collision with the windscreen. Find the ratio of v1/v2 .