Номер 356

420 + 40 + 100 = (400 + 20) + 40 + 100 = (400 + 100) + (20 + 40) = 500 + 60 = 560;
(500 + 30) + 60 = 500 + (30 + 60) = 590;
(70 + 20) + 900 = 90 + 900 = 990;
400 + (400 + 20) = (400 + 400) + 20 = 800 + 20 = 820;
600 + 200 + 40 = 800 + 40 = 840;
(300 + 50) + 500 = (300 + 500) + 50 = 800 + 50 = 850.