Ответ:x + 400 = 500; x = 500 − 400; x = 100.600 + x = 900; x = 900 − 600; x = 300.x − 300 = 400; x = 400 + 300; x = 700.800 − x = 200; x = 800 − 200; x = 600.700 − x = 200; x = 700 − 200; x = 500.x − 800 = 0; x = 0 + 800; x = 800.