A force of $50$ N acts on a stationary ball of mass $0.5$ kg for $0.2$ s. Find the velocity of the ball immediately after the impact.

Impulse equals change in momentum: $Ft = mv - 0$, so $50 \times 0.2 = 0.5v$, giving $v = \dfrac{10}{0.5} = 20$ m s$^{-1}$. Stopping at the impulse of 10 N s and reporting it as velocity is the usual error.