If a car can go from 20m/s to 40m/s in 4.0 secs, what would be it’s acceleration?

This is a problem that would be a good test of your understanding rather than your ability to work the formulas. 5m/s² means that the velocity increase each second is 5 m/s. So 4 s of that acceleration would increase the speed (in m/s) from 20 to 40. (Speed increase each second is 5 m/s. We need an increase of 20 m/s.)

Since the acceleration is uniform during those 4 s, we can use the simple average speed of 30 m/s. 30 m/s * 4 s = 120 m.