A ball of mass 0.7 kg strikes the wall at an angle of 90 degrees with speed 72 km/h. Consider that the bounce lasts for 0.1 s and is perfectly elastic. What is the magnitude of the average reaction force from the wall that acts on the ball?

initial speed: v =  72 km/h = 20 m/s. 
mass: m = 0.7 kg
time of the impact: t = 0.1 s
average force: F
The speed after impact is the same as initial speed, but velocity has got the opposite direction. Therefore, Δv = 2v

Δmomentum = Δp = mΔv = 2mv

from the Newton's 2nd Law:
F = Δp/Δt = 2mv/t
F = 20.720/(0.1) = 280 N

Answer: The magnitue of the average force equals 280 N.