Since I've only memorized a couple of formulas for these things, I need to do the time first:
I'll use the formula Distance = (1/2) (acceleration) (time)²
20 m = (1/2) (10 m/s²) (time)²
Time² = (20 m) / (5 m/s²)
Time² = 4 sec²
Time = 2 seconds
Now ...
-- When the ball hits the ground, it has been falling for 2 seconds.
-- Its acceleration has caused its speed to increase by 10 m/s every second.
-- So after 2 seconds, its speed has grown to (2 s) (10 m/s²) = 20 m/s .
