For this problem, we will need the famous equation F=ma, as detailed by Isaac Newton.
F=mass x acceleration, where Force is in Newtons.
20 m/s represents the quantity of speed change, but to use in the equation, we need acceleration, not speed. Acceleration is the velocity change over a unit time, so changing 20 m/s in 0.1 seconds means that the acceleration was 200 m/s^2.
Plugging in all your numbers, you find:
F=(0.45 kg)(200 m/s^2) = 90 N of force.
