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5分之7x+5分之7y=210 7(x+y)=5*210 x+y=150 5分之7x×0.8+5分之7y×0.9=? 0.56x+0.63y=5? 0.56(x+y)+0.7y=5? 0.56*150+0.7y=5? 0.7y=5?-84
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