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