If a 20 kg box is accelerating upward at 8 m/s^2 , what is the magnitude of the upward force

356 N

Explanation:

Newton's second law of motion describes the relationship between an object's mass and the amount of force needed to accelerate it.  

here there is a weight acting downwards on the object , yet it is accelerating  upwards shows that there is a net force upwards

Applying Newtons second law of motion upwards ,

Force  = mass *acceleration

Upward force - m*g = m *a

F_up= 20 *8 + 20 * 9.8

         = 356 N

Upward force= 160 N

because if the tool that you are using is not balanced it there would be false mass.