The product of two negatives and one positive will give you a negative number. Take the three numbers; -1, -1, and 1. There are two negatives and one positive. -1 x -1 x 1 = 1 x 1 = 1 = positive number So the possible answers are the positive numbers:
Multiplying two negatives results in a positive product. Multiplying this positive product by another positive number will result in a positive answer. This means the only possible answers from our choices are the two positive ones, 20 and 10.
