(900 - x : 6) * 5 = 4200
900 - x : 6 = 4200 : 5
900 - x : 6 = 840
-x : 6 = 840 - 900
-x : 6 = -60
-x = -60 * 6
-x = -360
x = 360
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Проверка
(900 - 360 : 6) * 5 = (900 - 60) * 5 = 840 * 5 = 4200
4200 = 4200
Ответ: 360
325 + (90 - n) : 17 = 330
(90 - n) : 17 = 330 - 325
(90 - n) : 17 = 5
90 - n = 5 * 17
90 - n = 85
-n = 85 - 90
-n = -5
n = 5
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Проверка
325 + (90 - 5) : 17 = 325 + 85 : 17 = 325 + 5 = 330
330 = 330
Ответ: 5