So, the initial speed of the ball as it leaves the jugglers hand is 20m/s
u = 20m/s
v = 0 (Since the ball will reach 0 velocity, once it reaches the maximum height possible and then will fall back again)
a = -g = -10m/s^2.
So, the time taken by ball to reach its maximum height
v = u+at
0 = 20-10t
t = 2 sec.
So, total time for which the ball will be in air will be 4 sec i.e. juggler will touch the same ball after 4 sec only. Since, he is juggling 4 balls at regular intervals, i.e. each of them is spaced at 1 sec distance interval.
Also, the maximum height to which the ball reaches
v^2 = u^2 + 2as
400 = 20s
s = 20m
hence, currently, when one ball leaves juggler's hand, other balls will be at 10 (going up), 20 and 10 (coming down) m respectively.