The mass is given of the ball is, $m=0.2 \mathrm{kg}$, now first calculate the retardation of the ball, using the first equation of the motion, 

$$\begin{gathered} v=u+at\Rightarrow 0=20+a\times 0.1 \\ \Rightarrow a=-200 \mathrm{m} {\mathrm{s}}^{-2} \end{gathered}$$

So the average force applied to the ball, $F=m\left|a\right|=0.2\times 200=40 \mathrm{N}$