用一元一次方程解答Mrs jones invests $4000 for 10% yearly profit and $6000 at 12%yearly profit .how much should she invest at 15%profit so that her total yearly profit is 13%of the total amount she has invest?
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Suppose she should invest $x at 15% profit
Then 4000*10%+6000*12%+x*15% = (4000+6000+x)*13%
So x = 9000