Зарание . 180-(3*х)*2=120. (х+15)*6=420.

1)
180 - (3*х)*2 = 120
(3*х)*2 = 180 - 120
(3*х)*2 = 60
3*х = 60 : 2
3*х = 30
х = 30 : 3
х = 10

180 - (3*10)*2 = 120
180 - 30*2 = 120
180 - 60 = 120
120 = 120
ответ: х = 10.

2)
(х + 15)*6 = 420
х + 15 = 420 : 6
х + 15 = 70
х = 70 - 15
х = 55

(55 + 15)*6 = 420
70 * 6 = 420
420 = 420
ответ: х = 55

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