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x + 320 = 80 × 7
x + 320 = 560
x = 560 - 320
x = 240
Проверка: 240 + 320 = 560 = 80 × 7
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x - 180 = 240 ∶ 3
x - 180 = 80
x = 80 + 180
x = 260
Проверка: 260 - 180 = 80 = 240 ∶ 3
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400 - x = 275 + 25
400 - x = 300
x = 400 - 300
x = 100
Проверка: 400 - 100 = 300 = 275 + 25
Ответ: 1) x = 240; 2) x = 260; 3) x = 100