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1.2600-280X=2120-160X
解:2600-2120=-160X+280X
480=120X
X=480÷120
X=4
2.X(1-20%)+3.5=X-4
解: X-0.2X+3.5=X-4
0.8X+3.5=X-4
7.5=0.2X
X=7.5÷0.2
X=37.5