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540:х+15=150:2
х*50+100*9=250*4
120+х*15=375
600-120+х*12=600
1260+12*х=3000

540 : х + 15 = 150 : 2

540 : х = 150 : 2 - 15

540 : х = 60

х = 540 : 60

х = 9

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х * 50 + 100 * 9 = 250 * 4

х * 50 = 1000 - 900

х * 50 = 100

х = 100 : 50

х = 2

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120 + х * 15 = 375

х * 15 = 375 - 120

х * 15 = 255

х = 255 : 15

х= 17

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600 - 120 + х * 12 = 600

х * 12 = 600 - 600 + 120

х * 12 = 120

х = 120 : 12

х = 10

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1260 + 12 * х = 3000

12 * х = 3000 - 1260

12 * х = 1740

х = 1740 : 12

х = 145