Answer
$6\space N$
Work Step by Step
Please see the attached image first.
We can write,
Change in momentum of ice ball $=\Delta m\vec V= m(V-u)$
$\Delta m\vec V= 0.3kg\times(0-10)m/s= -3\space kgm/s$
So we can write,
The change in momentum of the window = 3 kgm/s in one collision. But there are two collisions during one second on the wall.
$F=\frac{\Delta m\vec V}{\Delta t}= 2\times\frac{3kgm/s}{s}=6\space kgm/s^{2}$
Average force exerted on the window = 6N
