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Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu https://gmatclub.com/forum/bobjustfilledhiscarsgastankwith20gallonsofgasoholamixtu45790.html 
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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. 9/10 B. 1 C. 10/9 D. 20/19 E. 2 
Author:  Bunuel [ 11 Feb 2011, 16:25 ] 
Post subject:  Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 
gmatpapa 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) 9/10 b) 1 c) 10/9 d) 20/19 e) 2 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. Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace. 19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol > thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons > amount of the ethanol which Bob must add is 190/920=10/9 gallons. Answer: C. 
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 = 1 G (gasoline) = 20  1 = 19 Now, set up the new ratio that we are trying to obtain using the current one: 1/9 = (1 + x)/19 19 = 9 + 9x 10 = 9x x = 10/9 1/9 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) 9/10 b) 1 c) 10/9 d) 20/19 e) 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) = 1/10 The 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 500600 level question , but if you start using logic it becomes relatively a difficult question 600700 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 ethanol After adding x gallons of ethanol; the total weight of the fuel=20+x Amount of ethanol in it will be = 1+x (1+x) is 10% of (20+x) (1+x/20+x)*100=10 100+100x=200+10x 90x=100 x=10/9 Ans: "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% = 1 Let x liter ethanol has to be mixed 1+x = 0.10(20+x) 1+x = 2+0.10x 0.90x = 1 x= 10/9 Ans. 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. 9/10 B. 1 C. 10/9 D. 20/19 E. 2 It is better to apply formula of weighted average: V1/V2= (C2C3)/ (C3C1) V1 and V2=> initial and final volumes of mixture respectively C1= Initial concentation of fluid= 5% C2= Concentration of additive fluid= 100% ethanol in our case C3= Desired concentration of fluid= 10% 20/V2= (10010)/(105) 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 practice These 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:  ENAFEX [ 01 Oct 2012, 00:01 ] 
Post subject:  Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 
Bunuel wrote: gmatpapa 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) 9/10 b) 1 c) 10/9 d) 20/19 e) 2 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. Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace. 19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol > thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons > amount of the ethanol which Bob must add is 190/920=10/9 gallons. Answer: C. Bunuel, Help me with this. I am clear with this  19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol > thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons Now the total fuel is 190/9 gallons(Gasoline+ethanol). As given in the question ethanol will be 10% of this new solution which is 19/9. What is the mistake here? 
Author:  Bunuel [ 01 Oct 2012, 08:04 ] 
Post subject:  Re: Bob just filled his car's gas tank with 20 gallons of gasohol, a mixtu 
ENAFEX wrote: Bunuel wrote: gmatpapa 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) 9/10 b) 1 c) 10/9 d) 20/19 e) 2 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. Please read the question carefully: "how many gallons of ethanol must he add ..." add not replace. 19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol > thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons > amount of the ethanol which Bob must add is 190/920=10/9 gallons. Answer: C. Bunuel, Help me with this. I am clear with this  19 gallons of gasoline which is now in the tank must comprise 90% of the whole fuel after adding some amount of ethanol > thus whole amount of fuel after adding must be 19/(9/10)=190/9 gallons Now the total fuel is 190/9 gallons(Gasoline+ethanol). As given in the question ethanol will be 10% of this new solution which is 19/9. What is the mistake here? Yes, there will be 19/9 gallons of ethanol in the tank. But we are asked "how many gallons of ethanol must he add ..." and since there were 20*5%=1 gallon of ethanol initially, then we should add 19/91=10/9 gallons of ethanol. Hope it's clear. 
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%gasoline which is 19:1 Optimal 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 19/9/1020=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+0.9x 0.1x = 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+0.9x 0.1x = 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 = 1 x = 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. 9/10 B. 1 C. 10/9 D. 20/19 E. 2 Responding 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) = 18/1 For 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. 9/10 B. 1 C. 10/9 D. 20/19 E. 2 Responding 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) = 18/1 For 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. 9/10 B. 1 C. 10/9 D. 20/19 E. 2 Responding 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) = 18/1 For 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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