His average speed all the way is
(50 meters) / (5 sec) = 10 m/s .
But, if the acceleration is uniform all the way down, then
Average speed = (1/2) (start speed + end speed)
Start speed = 0.
So
Average speed = (1/2) (0 + end speed)
= (1/2) of end speed .
10 m/s = (1/2) of end speed
End speed (at the bottom) = 20 m/s .
Magnitude of acceleration = (change in speed) / (time for the change)
= (20 m/s) / (5 sec)
= 4 m/s² .
