Rank the six combinations of electric charges on the basis of the electric force acting on q1. Define forces pointing to the right as positive and forces pointing to the left as negative. Rank positive forces as larger than negative forces. Rank from largest to smallest, placing the largest on the left and the smallest on the right. To rank items as equivalent, overlap them. The correct ranking cannot be determined. There are two types of charge, which have opposite signs. Which type we call "positive" is an arbitrary convention. Therefore, if all of the charges in a combination changed sign, the magnitude and direction of the electric force would not change. Look for pairs of combinations where all of the charges in one have the opposite signs of the charges in the second. To show this equality, stack the cards for the equivalent charge combinations one on top of another.