{X+Y=25 （1）y=25-x
{ 4200X+3100Y=100000
4200x+3100(25-x)=100000;
42x+31×25-31x=1000;
11x=25×9;
x=225/11;
y=25-225/11=275/11-225/11=50/11;
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{X+Y=25 { 4200X+3100Y=100000 {X+Y=25 {3700X+3100Y=100000
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