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Bob just filled his car's gas tank with 20 gallons of

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Director
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Bob just filled his car's gas tank with 20 gallons of [#permalink] New post 20 May 2007, 21:52
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Bob just filled his car's gas tank with 20 gallons of gasohol, a mixture consisting of 5% ethanol and 95% gasoline. If his car runs best on a mixture consisting of 10% ethanol and 90% gasoline, how many gallons of ethanol must he add into the gas tank for his car to achieve optimum performance?
Director
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 [#permalink] New post 20 May 2007, 21:59
Answer = 10/9 or 1.11 G of ethanol must be added.

In tank = 20G of fuel => 1G ethanol / 19G gasoline

expected 10% ethanol,

(10/100) (20 + x ) = 1 + x => x = 1.1
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 [#permalink] New post 20 May 2007, 22:19
U could do algebra or could have realized this. 95%=19 and 5%=1

If you add 1 more G of E then you have 21 gals total... almost 10% E, but not quite. b/c 10% of 21 is 2.1. So u would realize that u need to add .1. Well from here it just keeps repeating .01, .001, .0001, etc... Just look in the answer choices to see where it looks close to that.

It should be 10/9


Or u could have done it this way: .10(19/.9) -1= 10/9 or 1.11111...

this problem was on an ealier post
  [#permalink] 20 May 2007, 22:19
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