.

Force exerted by each ball is $F=\frac{∆p}{∆t}$, here, $p$ is momentum and $t$ is time.

So, for one ball $F=\frac{mv-m\left(-v\right)}{t}=\frac{2mv}{t}$

Now, total force exerted by the balls on the wall is $F_{net}=100\times F=\frac{200mv}{t}$.