A stone is dropped from top of a tower of 50m high, simltaneously another stone is thrown upward with a speed of 20m/s find the time when them cross each other

At the time that I'll call ' Q ', the height of the stone that was

dropped from the tower is

H = 50 - (1/2 G Q²),

and the height of the stone that was tossed straight up

from the ground is

H = 20Q - (1/2 G Q²).

The stones meet when them's heights are equal,

so that's the time when

50 - (1/2 G Q²) = 20Q - (1/2 G Q²).

This is looking like it's going to be easy.

Add (1/2 G Q²) to each side.

Then it says

50 = 20Q

Divide each side by 20: 2.5 = Q.

And there we are. The stones pass each other

2.5 seconds

after they are simultaneously launched.