Условие:

70 * 12
70 * 20
15 * 13
15 * 30

70 * 12 = 70 * (10 + 2) = 70 * 10 + 70 * 2 = 700 + 140 = 840;
70 * 20 = 70 * (2 * 10) = (70 * 2) * 10 = 140 * 10 = 1400;
15 * 13 = 15 * (10 + 3) = 15 * 10 + 15 * 3 = 150 + 45 = 195;
15 * 30 = 15 * (3 * 10) = (15 * 3) * 10 = 45 * 10 = 450.