An object of mass 1000 kg, initially having a velocity of 100 m/s, decelerates to a final velocity of 20 m/s. what is the change in kinetic energy of the object, in kj?

M = 1000 kg
v_in = 100 m/s
v_f = 20 m/s

delta KE = 1/2 m (v_in)^2 - 1/2 m * (v_f)^2
delta KE = 1/2 1000 (100 m/s)^2 - 1/2 1000 (20 m/s)^2
delta KE = 1/2 1000 [ 10000 m^2/s^2 -  400 m^2/s^2
delta KE = 500 * 9600 m^2/s^2
delta KE = 4800000 Joules or 4800 kJ

Note: you could do this without taking out 1/2 1000 as a common factor in both terms. You would get exactly the same answer. Perhaps that is how I should have done it.