Давай решим примеры пошагово:
* 39 * 12 = 39 * (10 + 2) = 39 * 10 + 39 * 2 = 390 + 78 = **468**
* 72 * 20 = 72 * (2 * 10) = 72 * 2 * 10 = 144 * 10 = **1440**
* 40 * 35 = 40 * (30 + 5) = 40 * 30 + 40 * 5 = 1200 + 200 = **1400**
* 152 * 30 = 152 * (3 * 10) = 152 * 3 * 10 = 456 * 10 = **4560**