X be the distance from the ground where the collision takes place then
Distance from top of tower up to the collision point =40-X
let collision took place after time t then
Adding (1) and (2) we get
Which gives t=1 second
Putting value of t in (1) we get
( taking :g= 9.8 m /s^2)
So collision takes at 15.1 metres from ground or 40–15.1=24.9 metres from top.