A ball is thrown upward at 20 m/s from the top of a building, which is 100 meter high. Another ball is thrown downward from the top of the building at 15 m/s. There is a window on the building, which is 2.5 meters high and the top of the window is 60 meters from the ground.

a) How many seconds later will the upward thrown ball hit the ground after the downward thrown ball?

b) What is their final speed when they hit the ground?

c) How many seconds will each ball be visible through the window?

d) If the above building is 100 meters above a lake, which is 60 meters deep, and the balls sink at constant velocity upon entering the lake, how many total seconds will it take each ball to reach the bottom of the lake?