2 tall buildings are 200m apart. with what speed a ball be thrown horizontally from a window of one building 2km above the ground so that it will enter a window 40m from the ground in the other?

Answer 1

2000 meters - 40 meters = 1960 meter fall

vertical fall problem first
1960 = 4.9 t^2
t = 20 second fall

so it has to go 200 meters horizontal in 20 seconds
u = 200/20 = 10 m/s