A stone is allowed to fall from the top of a tower 100m high and at the same time another stone is projected vertically upwards from the ground with a velocity of 25m/s. Calculate when and where the two stones will meet.

Let the stones meet at point A after time t.
For upper stone : u'=0
x=0+1/2gt²
x=1/2×10×t²⟹x=5t²(1)
For lower stone : u=25m/s
100-x=ut-1/2gt²
100-x=(25)t-1/2×10×t²⟹100-x=25t-5t²(2)
Adding (1) and (2), we get
25t=100⟹t=4 s
From (1),
x=5×4²⟹x=80 m
Hence the stone meet at a height of 20m above the ground after 4 seconds.