Answer
$F_{avg}=68.4N$
Work Step by Step
We can find the average force as follows:
First of all, we will find the velocity when the ball hits the ground
$v^2=v_i^2+2gh$ (Initially, the ball falls from rest therefore $v_i=0$)
$\implies v^2=2gh$
$\implies v=\sqrt{2gh}$
We plug in the known values to:
$v=\sqrt{2(9.8)(15)}=17.1m/s$
We take $v=-17.1m/s$ because the ball is falling downward.
Now $F_{avg}=\frac{m(v_f-v_i)}{\Delta t}$
We plug in the known values to obtain:
$F_{avg}=\frac{2(0-(-17.1))}{0.5}$
This simplifies to:
$F_{avg}=68.4N$