1.) Velocity is in m/s, and acceleration is in m/s^2 like you said. Because of this, we can calculate this by dividing the speed by the time it took to get to that speed. (20 meters/second) / 10 seconds = 2 meters/ second^2
2.) Same thing with the first one. (100 meters/second) / 4 seconds = 25 meters / seconds^2
We know that acceleration can increase depending in the force applied on an object, any object with a greater mass will apply a greater force. F = M(a).