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

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Bob just filled his car's gas tank with 20 gallons of [#permalink]

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18 May 2007, 15:38
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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?

A. 9/10
B. 1
C. 10/9
D. 20/19
E. 2
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Re: Word Problem - chemical mixture [#permalink]

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18 May 2007, 20:35
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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.
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18 May 2007, 23:11
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its c. Lets say you add x gallons of ethanol more. Than
(1+x)=(20+x)/10

x=10/9
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Re: Word Problem - chemical mixture [#permalink]

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20 Jun 2009, 10:37
It's C.

Using the table:
concent 5% 100% 10%
amt 20 x 20+x
= 1 + x = 2 + .10x
= 10/9
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Re: Word Problem - chemical mixture [#permalink]

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22 Jun 2009, 09:27
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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
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Re: Word Problem - chemical mixture [#permalink]

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23 Jun 2009, 07:09
Arvindgmat wrote:
What do you think is the difficulty level of the question. I did solve it but took a lot of time to find the answer. Took a couple of wrong approaches to solve the problem.

i am guessing medium difficulty. Pretty straightforward if someone done the type of question before.
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Re: Word Problem - chemical mixture [#permalink]

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23 Jun 2009, 08:47
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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..
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Re: Word Problem - chemical mixture [#permalink]

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24 Jun 2009, 11:14
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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%.
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Re: Word Problem - chemical mixture [#permalink]

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11 Feb 2011, 15:11
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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.
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Re: Word Problem - chemical mixture [#permalink]

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11 Feb 2011, 15:25
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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.

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 190/9-20=10/9 gallons.

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Re: Word Problem - chemical mixture [#permalink]

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12 Feb 2011, 00:42
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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"
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Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

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14 Dec 2011, 11:23
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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
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Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

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05 Apr 2012, 05:30
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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= (C2-C3)/ (C3-C1)

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= (100-10)/(10-5)
V2= 100/90= 10/9
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05 Apr 2012, 08:48
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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: http://www.veritasprep.com/blog/2011/04 ... -mixtures/

Also, you should get many such problems under the topic 'mixtures' in any standard GMAT prep book.
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Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Manager Status: And the Prep starts again... Joined: 03 Aug 2010 Posts: 138 Followers: 2 Kudos [?]: 48 [0], given: 20 Re: Word Problem - chemical mixture [#permalink] ### Show Tags 30 Sep 2012, 23:01 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 190/9-20=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? _________________ My First Blog on my GMAT Journey Arise, Awake and Stop not till the goal is reached Math Expert Joined: 02 Sep 2009 Posts: 36609 Followers: 7099 Kudos [?]: 93523 [1] , given: 10568 Re: Word Problem - chemical mixture [#permalink] ### Show Tags 01 Oct 2012, 07:04 1 This post received KUDOS Expert's post 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 190/9-20=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 19/9-1=10/9 gallons of ethanol. Hope it's clear. _________________ Intern Joined: 03 May 2012 Posts: 5 Followers: 1 Kudos [?]: 12 [3] , given: 4 Re: Bob just filled his car's gas tank with 20 gallons of [#permalink] ### Show Tags 22 Nov 2012, 19:56 3 This post received KUDOS 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 19/9/10-20=10/9 Intern Joined: 09 Jan 2012 Posts: 11 Followers: 0 Kudos [?]: 0 [0], given: 4 Re: Bob just filled his car's gas tank with 20 gallons of [#permalink] ### Show Tags 01 Jan 2013, 13:34 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 ??? Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7130 Location: Pune, India Followers: 2140 Kudos [?]: 13703 [1] , given: 222 Re: Bob just filled his car's gas tank with 20 gallons of [#permalink] ### Show Tags 01 Jan 2013, 20:51 1 This post received KUDOS Expert's post 1 This post was BOOKMARKED 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 _________________ Karishma Veritas Prep | GMAT Instructor My Blog Get started with Veritas Prep GMAT On Demand for$199

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Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

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01 Jan 2013, 23:11
That's so cool, thanks so much Karishma, it's clear now !
Re: Bob just filled his car's gas tank with 20 gallons of   [#permalink] 01 Jan 2013, 23:11

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