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 Author: above720 [ 18 May 2007, 16:38 ] Post subject: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 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?A. 9/10B. 1C. 10/9D. 20/19E. 2

 Author: smarinov [ 24 Jun 2009, 12:14 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu My vote is for C.First find the current ratio:E (ethanol) = 5% x 20 = 1G (gasoline) = 20 - 1 = 19Now, set up the new ratio that we are trying to obtain using the current one:1/9 = (1 + x)/19 19 = 9 + 9x10 = 9xx = 10/91/9 represents the new ratio we are trying to get of 10% ethanol and 90% gasoline. x is for the amount of gallons we need to add to the current 1 gal. of ethanol to achieve the new 10%.

 Author: Himalayan [ 18 May 2007, 21:35 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu above720 wrote: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?a) 9/10b) 1c) 10/9d) 20/19e) 2 E = 5% of 20 gl = 1 gl G = 95% of 20 gl = 19 we need to add E. if so, added E = 10% of (19/.9) - 1 = 10/9 gl C.

 Author: CrackTheGmatInSFO [ 19 May 2007, 00:11 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu its c. Lets say you add x gallons of ethanol more. Than (1+x)=(20+x)/10 x=10/9

 Author: shaselai [ 22 Jun 2009, 10:27 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu I also agree it is C.you need to go from 1/20 to (1+x)/(20+x) = 1/10The trap answer is 1 since 1+1=2/20 =1/10 when it really is 2/21

 Author: sanjaysingh82 [ 23 Jun 2009, 09:47 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu the solution is simple.. if your try using simple maths its an easy question 500-600 level question , but if you start using logic it becomes relatively a difficult question 600-700 level..mathematical way: let x litres be added,, so x+1/ 20+x = 1/10 and find x easy as every one else has done here........ but if you start to use logic.. you will end up eating too much time..

 Author: gmatpapa [ 11 Feb 2011, 16:11 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu Is it that we dont't need to have 20 litres of gasohol? Because if we do, I think the answer will be B. Currently there is 5% of ethanol out of 20 litres of fuel that means the mixture contains 1 litre of ethanol. To have 10% ethanol, Bob needs to have 2 litres of ethanol, therefore he needs to add 1 more litre of ethanol.

 Author: fluke [ 12 Feb 2011, 01:42 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu Originally; the fuel contained 20*5/100 = 1 gallon of ethanolAfter adding x gallons of ethanol; the total weight of the fuel=20+xAmount of ethanol in it will be = 1+x(1+x) is 10% of (20+x)(1+x/20+x)*100=10100+100x=200+10x90x=100x=10/9Ans: "C"

 Author: MHIKER [ 14 Dec 2011, 12:23 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 5% ethanol = 20*5% = 1Let x liter ethanol has to be mixed1+x = 0.10(20+x)1+x = 2+0.10x0.90x = 1x= 10/9Ans. C

 Author: Cmplkj123 [ 05 Apr 2012, 06:30 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu above720 wrote: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?A. 9/10B. 1C. 10/9D. 20/19E. 2It is better to apply formula of weighted average:V1/V2= (C2-C3)/ (C3-C1)V1 and V2=> initial and final volumes of mixture respectivelyC1= Initial concentation of fluid= 5%C2= Concentration of additive fluid= 100% ethanol in our caseC3= Desired concentration of fluid= 10%20/V2= (100-10)/(10-5)V2= 100/90= 10/9

 Author: KarishmaB [ 05 Apr 2012, 09:48 ] Post subject: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu TomB wrote: does anybody know some other problems like above problems? I need some extra practiceThese are standard mixture problems. I have discussed a couple of such problems on my blog (link below).Also, you should get many such problems under the topic 'mixtures' in any standard GMAT prep book.

 Author: chanchalkalra [ 22 Nov 2012, 20:56 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu Total -20 gallon gasohol.95% ethanol and 5%gasolinewhich is 19:1Optimal mixture should be 18:2 as per details given.Now,19 gallon of ethanol should be equal to 90%.So for that you should add more than 1 gallon of gasoline.i.e 19/9/10-20=10/9

 Author: Emaco [ 01 Jan 2013, 14:34 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu Guys, I made the following formula, could anyone of you please tell me what's wrong with it ?0.95 (20) + X = 90/100 (20+x)1+X = 18+0.9x0.1x = 17 ???

 Author: KarishmaB [ 01 Jan 2013, 21:51 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu Emaco wrote:Guys, I made the following formula, could anyone of you please tell me what's wrong with it ?0.95 (20) + X = 90/100 (20+x)1+X = 18+0.9x0.1x = 17 ???.95 of 20 is the amount of gasoline already in his car. He adds more ethanol and not gasoline so you cannot add x (amount of ethanol added) to .95 of 20.Instead, you need to find the current amount of ethanol and add x to it..05(20) + x = .10 (20 + x).9x = 1x = 10/9

 Author: KarishmaB [ 05 Aug 2013, 22:54 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu above720 wrote: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?A. 9/10B. 1C. 10/9D. 20/19E. 2Responding to a pm:Using Weighted average formula:w1/w2 = (A2 - Avg)/(Avg - A1)You need to mix 5% ethanol mixture with 100% ethanol to give a 10% ethanol mixture.w1/w2 = (100 - 10)/(10 - 5) = 18/1For every 18 parts of gasohol, you need to put 1 part of pure ethanol.So if you have 20 gallons of gasohol, you need to put 20/18 (= 10/9) gallons of pure ethanol.Answer (C)

 Author: rrsnathan [ 14 Aug 2013, 05:49 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu VeritasPrepKarishma wrote:above720 wrote: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?A. 9/10B. 1C. 10/9D. 20/19E. 2Responding to a pm:Using Weighted average formula:w1/w2 = (A2 - Avg)/(Avg - A1)You need to mix 5% ethanol mixture with 100% ethanol to give a 10% ethanol mixture.w1/w2 = (100 - 10)/(10 - 5) = 18/1For every 18 parts of gasohol, you need to put 1 part of pure ethanol.So if you have 20 gallons of gasohol, you need to put 20/18 (= 10/9) gallons of pure ethanol.Answer (C)Hi,Is this alligation rule method?Regrds,Rrsnathan.

 Author: KarishmaB [ 15 Aug 2013, 22:42 ] Post subject: Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu rrsnathan wrote:VeritasPrepKarishma wrote:above720 wrote: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?A. 9/10B. 1C. 10/9D. 20/19E. 2Responding to a pm:Using Weighted average formula:w1/w2 = (A2 - Avg)/(Avg - A1)You need to mix 5% ethanol mixture with 100% ethanol to give a 10% ethanol mixture.w1/w2 = (100 - 10)/(10 - 5) = 18/1For every 18 parts of gasohol, you need to put 1 part of pure ethanol.So if you have 20 gallons of gasohol, you need to put 20/18 (= 10/9) gallons of pure ethanol.Answer (C)Hi,Is this alligation rule method?Regrds,Rrsnathan.Yes, alligation uses a diagram (with a cross with average in the middle) which leads to this formula. This formula is very effective in solving most weighted average/mixture problems.

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