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設進價為X,標價為Y,立方程式如下: Y*0.9=X+20 Y*0.7=X-10 合併以上: 1.6Y=2X+10 X=0.8Y-5 代入上面: 0.9Y=0.8Y-5+20 0.1Y=15 Y=150 再次代入上面: X=120-5 X=115 該商品的進價115元和標價150元.
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