Time of ascent = Time of descent.
t1 + t2 = 4
t2 + t2 = 4
t2 = 2 sec
Now, taking vertical downward motion of the ball from the highest point to surface of earth we have,
u = 0, a = 10 m/s, t = 2 s, s = ?
Now, using the formula,
s = ut + 1/2 at2
= 20 m
This is the height to which the ball has risen.