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i think level of this question would be 640-650

1. in 20 gallons of mixture 5% is ethanol, that would be 1 gallon, and rest of 19 gallon is gasoline.
2. no we need to add ethanol so that the composition of ethanol in the mixture would become 10%.
3. considering 19 gallon of gasoline in the mixture to be 90%, we can figure out how much is the total volume of the gasohol -- x*(90/100)=19, x will come out to be 190/9.
5. now subtract 19 out of 190/9, it will be 19/9.
6. out of 19/9 gallons of ethanol 1 gallon was already there in the mixture, subtract 1 out of 19/9
19/9-1 = 10/9
and that is your answer..

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0.1(20+x)=0.05*20+x, multiply both sides by 100, 200+10x=100+100x, 90x=100, x=100/90=10/9

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(20*0.05+x)/(20+x)=1/10

(1+x)/(20+x)=1/10
10+10x=20+x
9x=10
x=10/9

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