Author 
Message 
TAGS:

Hide Tags

Intern
Joined: 31 Oct 2012
Posts: 13

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
02 Jan 2013, 12:01
In the original mixture we have 95% G= 19 gallons and 5% E= 1 gallon We have to add X amount of Ethanol to make it 90:10 mixture, hence: (1+x/20+x)*100=10 (1+x)*10=20+x 9x=10; x=10/9 thus the answer is C
Last edited by nk9285 on 02 Jan 2013, 12:18, edited 1 time in total.



Manager
Joined: 24 Apr 2013
Posts: 53

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
20 May 2013, 15:17
I got 1 cause i got 5% to be 1 and the reason i forgot that is because i forgot to 95% gasoline



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7449
Location: Pune, India

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
05 Aug 2013, 22:54
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)
_________________
Karishma Veritas Prep  GMAT Instructor My Blog
Get started with Veritas Prep GMAT On Demand for $199
Veritas Prep Reviews



Manager
Joined: 30 May 2013
Posts: 188
Location: India
Concentration: Entrepreneurship, General Management
GPA: 3.82

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
14 Aug 2013, 05:49
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.



Senior Manager
Joined: 10 Jul 2013
Posts: 334

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
15 Aug 2013, 04:38
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 Ethanol now = 5% of 20 = 1 gallon Let, we need x gallons of ethanol . so, (20+x)/(1+x) = 100/10 or, x = 10/9 (Answer)
_________________
Asif vai.....



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7449
Location: Pune, India

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
15 Aug 2013, 22:42
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.
_________________
Karishma Veritas Prep  GMAT Instructor My Blog
Get started with Veritas Prep GMAT On Demand for $199
Veritas Prep Reviews



Intern
Joined: 03 May 2013
Posts: 4

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
19 Sep 2013, 04:50
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 Let the total gallons = 100. therefore, it has 5 gallon ethanol and 95 gallon gasoline and we need to make the mixture of 10:90 instead. Let the volume of ethanol added be 'x' in order to make the ratio 10:90. therefore, (5+x)/95 = 10/90. x = 50/9. Now, if in 100 gallons of fuel we need to add, 50/9 gallons of ethanol to make the ratio 10:90 In 20 gallons we would add = (50/9)/100 X 20 (simple unitary method) = 10/9 (Ans)



GMAT Club Legend
Joined: 09 Sep 2013
Posts: 16026

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
19 Sep 2014, 08:56
Hello from the GMAT Club BumpBot! Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up  doing my job. I think you may find it valuable (esp those replies with Kudos). Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
GMAT Books  GMAT Club Tests  Best Prices on GMAT Courses  GMAT Mobile App  Math Resources  Verbal Resources



Intern
Joined: 09 Feb 2014
Posts: 8

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
19 Nov 2014, 05:17
Let x is the gallons of ethanol to be added in the mixture => x has 100% ethanol. Totally ignore the information about gasoline because we're dealing with ethanol now.
The new mixture should be 20 + x (gallons) and have 10% ethanol => (20+x)*10% Add x gallons of 100% ethanol to 20 gallons of 5% ethanol => x*100% + 20*5% = x+1
We have the equation: (20 + x)*0,1 = x + 1 => x = 10/9



Intern
Joined: 05 Jan 2014
Posts: 5
Concentration: General Management, Technology
GPA: 3.28

Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
01 Dec 2014, 18:08
initial 20 = 1E + 19G
new 20+E = (20+E)/10 +19 (as amount of G same only E added)
=> 18 +9E/10 = 19
HENCE E = 10/9



SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1857
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
02 Dec 2014, 02:46
Ethanol ............... Gasoline .............. Total 1 ............................. 19 ..................... 20 (Current fill) 1+x ........................... 19 ................... 20+x (Say "x" litres of ethanol is added to make it 10%) Equation can be setup in two ways\(\frac{10}{100} (20+x) = 1+x\) OR \(\frac{90}{100} (20+x) = 19\) For both, computation of x would be the same \(x = \frac{10}{9}\) Answer = C
_________________
Kindly press "+1 Kudos" to appreciate



Manager
Joined: 13 Dec 2013
Posts: 55
GPA: 2.71

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
11 Dec 2014, 07:40
VeritasPrepKarishma wrote: 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 I solved it using algebra, for concepts sake, is it possible to use alligation or the scale method here?



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7449
Location: Pune, India

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
14 Dec 2014, 04:15
saadis87 wrote: I solved it using algebra, for concepts sake, is it possible to use alligation or the scale method here? In this post above: bobjustfilledhiscarsgastankwith20gallonsof4579020.html#p1253824I have already shown how to use scale method here. For more on the formula, check: http://www.veritasprep.com/blog/2011/03 ... averages/
_________________
Karishma Veritas Prep  GMAT Instructor My Blog
Get started with Veritas Prep GMAT On Demand for $199
Veritas Prep Reviews



Intern
Joined: 22 Jul 2014
Posts: 10

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
21 Aug 2015, 13:32
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 Using the cross method works out the best way to solve mixtures problems. But in this problem the final conversion from 18 parts to the asked 20 parts was something that tricked me. The silliness struck me after i solved it algebraically. \(Soln:\) Total amount: 20 Ethanol = \(\frac{5}{100}*20=1gl\) Gasoline = \(\frac{95}{100}*20=19gl\) Let \(x\) be the amount of ethanol that will be added to get 10% of ethanol in the final solution. => (1 + \(x\)) = \(\frac{10}{100}\)\(*(20+x)\) => 100 + 100\(x\) = 200 + 10\(x\) => 90\(x\) = 100 => \(x\) =\(\frac{10}{9}\) Therefore, \(\frac{10}{9}\) would be the amount of ethanol that needs to be added to \(20gl\) of fuel. Regards, Pratik
_________________
 The race is on .. ..
Consider to give a kudo if the post helped ! Why Kudos: http://gmatclub.com/forum/kudospointsongmatclubforum79704.html



Current Student
Joined: 02 Jun 2015
Posts: 12

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
21 Aug 2015, 19:49
I solved it a different way. It took me about 3 mins so I think I might be adding an extra step but this is what worked for me.
I started by solving how many gallons of ethanol and gas are in the tank already. .05 * 20=1 (gal of e); .95 * 20=19 (gal of g)
The current ratio of e:g is 1/19. The ideal ratio is 10/90 or 1/9. This question asks how much more ethanol we need to get to the 1/9 ratio.
Thus, e/19 = 1/9. > 9e=19. e=19/9 total. As there is already 1 gal of e in there, 19/9  (9/9)= 10/9 more gal of e.



Intern
Joined: 15 Sep 2012
Posts: 4
Location: India
Concentration: Operations, Technology
GPA: 2.9
WE: Information Technology (Consulting)

Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
23 May 2016, 21:22
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 how about this: E:G 5%:95% >> 1:19 19 is 90% of 190/9. 10% of 190/9 is 19/9. so we need : 19/99/9 = 10/9 gallons more.



Director
Joined: 18 Oct 2014
Posts: 908
Location: United States
GPA: 3.98

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
24 May 2016, 06:34
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 Let x gallon ethanol is added, making the total solution= 20+x 5% of 20 + x = 10%(20+x) 1+x= 2 + x/10 xx/10= 1 x= 10/9 C is the answer
_________________
I welcome critical analysis of my post!! That will help me reach 700+



Manager
Joined: 23 Dec 2013
Posts: 205

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
23 May 2017, 21:01
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 This problem involves weighted averages, but there isn't a shortcut perse. Instead, you should take the the current amount (1 gallon) plus x, the incremental amount of gas, and set this quantity equal to (x+20)/10, a tenth of the new total once the incremental gas has been added. This righthand quantity is the ideal amount. After this point, it's easy algebra to determine that x = 10/9.



Target Test Prep Representative
Status: Head GMAT Instructor
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 1082

Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]
Show Tags
26 May 2017, 11:47
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 We can let x = the number of gallons of ethanol to be added to the gas tank. Note that since the 20gallon mixture of gasohol is 5% ethanol, there is 20 x 0.05 = 1 gallon of ethanol already in the tank. Since we want the gasohol to be 10% ethanol, we have the following equation: (1 + x)/(20 + x) = 10/100 (1 + x)/(20 + x) = 1/10 10(1 + x) = 20 + x 10 + 10x = 20 + x 9x = 10 x = 10/9 Answer: C
_________________
Jeffery Miller
Head of GMAT Instruction
GMAT Quant SelfStudy Course
500+ lessons 3000+ practice problems 800+ HD solutions




Re: Bob just filled his car's gas tank with 20 gallons of
[#permalink]
26 May 2017, 11:47



Go to page
Previous
1 2
[ 39 posts ]




