The time interval between the object to the same point = 4 s
It is understood that at every 2-second interval, the ball reached the same height twice.
The velocity of the ball, v = u + (-gt)
0 = u – 10 x 2
u = 20
It is also given that at velocity 25m, time is 20 seconds.
Therefore, the initial velocity is, -v2 = u2 + (-2gh)
200 = u2 – 2 x 10 x 25
u2 = 900
Therefore, u = 30 m.s-1