Частина 2. Номер 523

120 • 4 = (100 + 20) • 4 = 100 • 4 + 20 • 4 = 400 + 80 = 480;

250 • 3 = (200 + 50) • 3 = 200 • 3 + 50 • 3 = 600 + 150 = 750;

360 • 2 = (300 + 60) • 2 = 300 • 2 + 60 • 2 = 600 + 120 = 720;

420 • 2 = (400 + 20) • 2 = 400 • 2 + 20 • 2 = 800 + 40 = 840.