Question

Mary wants to throw a can straight up in air and then hits it with second can.she the collision to occur at height h=10m above the throw point. In addition,she knows that she needs t1=4.0s between successive throws. Assume that she throws both cans with same speed.Take g=9.81m/^2. How long it takes after the first can has been thrown into air for the two cans to collide?