h= 50t - 5t^2
We have to calculate time when h=80, on substituting the value of h in above equation we get:
80 = 50t - 5t^2
or t^2 - 10t + 16 =0
so we get, t = 2 or 8
So at time t = 2 seconds or 8 seconds the ball will be at the height of 80 m
2 seconds when the ball is going up.
8 seconds when the ball is falling down.