The velocity of a car changes from 20 m/s east to 5 m/s east in 5 seconds. What is the acceleration of the car?

To solve this remember a=change in v/t, assuming the direction for the velocity, is + positive east. You can calculate it simply by this. A=v/t = A=5-20/5 = A=-15/5 = A=-3m/s squared. Change in velocity = vf-vi.