An object accelerates from rest to a velocity of 4.om / s over a distance of 20m . what was it's acceleration

Acceleration is the change in velocity over time. There's are formula's for such problems which are called kinematics equations. Look em up. One of them doesn't have t (time) in them you would normally use that one to solve this problem. 16/2/20=a=0.4m/s^2

I did it for you but please look it up, understand and memorize it ;)