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2x + 4(100-x) = 280

2x+400-4x=280

2x-4x=280-400

-2x=-120

x=-120/-2

x=60

Check

2x+4(100-x)

=2(60)+4(100-60)

=120+400-240

=520-240

=280 Right


I hope this can help you

2x+400-4x = 280
2x-4x = 280-400
-2x = -120
x =60