Since the problem starts with a negative number, you need to find out how many units from 0 it is. Since it is 20 units away from 0 in the negative direction, you need to add 20 in the positive direction to make it 0. Subtracting a negative number makes it positive. 2 negatives make a positive, so the problem is stated (rearranged) : 20 - 20 = 0.
