If a box of mass 20kg is being pulled by a force of 50N, and the friction is 10N, what is the resulting acceleration of the box?

First, we resolve the forces. So, if we take the positive direction to be the direction in which it is pulled, we get a force of +50N, and the friction goes against the motion, so the force of friction is -10N. So the sum of these two forces is 50N - 10N = 40N. Remembering Newton's Second Law of motion, F = ma, we know that the force = 40N, and mass = 20 kg, so acceleration a = F/m = 40N/20kg = 2 ms^-2.