It is currently 14 Dec 2017, 05:26

# Decision(s) Day!:

CHAT Rooms | Wharton R1 | Stanford R1 | Tuck R1 | Ross R1 | Haas R1

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

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

Author Message
TAGS:

### Hide Tags

Intern
Joined: 31 Oct 2012
Posts: 13

Kudos [?]: [0], given: 19

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

### Show Tags

02 Jan 2013, 11:01
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

Last edited by nk9285 on 02 Jan 2013, 11:18, edited 1 time in total.

Kudos [?]: [0], given: 19

Manager
Joined: 24 Apr 2013
Posts: 51

Kudos [?]: 11 [0], given: 76

Schools: Duke '16
Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

### Show Tags

20 May 2013, 14:17
I got 1 cause i got 5% to be 1 and the reason i forgot that is because i forgot to 95% gasoline

Kudos [?]: 11 [0], given: 76

Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7796

Kudos [?]: 18128 [0], given: 236

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

### Show Tags

05 Aug 2013, 21:54
Expert's post
2
This post was
BOOKMARKED
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.
_________________

Karishma
Veritas Prep | GMAT Instructor
My Blog

Get started with Veritas Prep GMAT On Demand for $199 Veritas Prep Reviews Kudos [?]: 18128 [0], given: 236 Manager Joined: 30 May 2013 Posts: 186 Kudos [?]: 90 [0], given: 72 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, 04:49 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. Kudos [?]: 90 [0], given: 72 Senior Manager Joined: 10 Jul 2013 Posts: 326 Kudos [?]: 430 [0], given: 102 Re: Bob just filled his car's gas tank with 20 gallons of [#permalink] ### Show Tags 15 Aug 2013, 03:38 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..... Kudos [?]: 430 [0], given: 102 Veritas Prep GMAT Instructor Joined: 16 Oct 2010 Posts: 7796 Kudos [?]: 18128 [0], given: 236 Location: Pune, India Re: Bob just filled his car's gas tank with 20 gallons of [#permalink] ### Show Tags 15 Aug 2013, 21:42 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

Kudos [?]: 18128 [0], given: 236

Intern
Joined: 03 May 2013
Posts: 4

Kudos [?]: [0], given: 48

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

### Show Tags

19 Sep 2013, 03:50
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)

Kudos [?]: [0], given: 48

Non-Human User
Joined: 09 Sep 2013
Posts: 14842

Kudos [?]: 287 [0], given: 0

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

### Show Tags

19 Sep 2014, 07:56
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.
_________________

Kudos [?]: 287 [0], given: 0

Intern
Joined: 09 Feb 2014
Posts: 8

Kudos [?]: 5 [0], given: 25

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

### Show Tags

19 Nov 2014, 04:17
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

Kudos [?]: 5 [0], given: 25

Intern
Joined: 05 Jan 2014
Posts: 5

Kudos [?]: 18 [0], given: 18

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, 17:08
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

Kudos [?]: 18 [0], given: 18

SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1849

Kudos [?]: 2788 [0], given: 193

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, 01:46
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

$$x = \frac{10}{9}$$

_________________

Kindly press "+1 Kudos" to appreciate

Kudos [?]: 2788 [0], given: 193

Manager
Joined: 13 Dec 2013
Posts: 53

Kudos [?]: 7 [0], given: 21

GPA: 2.71
Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

### Show Tags

11 Dec 2014, 06:40
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?

Kudos [?]: 7 [0], given: 21

Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 7796

Kudos [?]: 18128 [1], given: 236

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

### Show Tags

14 Dec 2014, 03:15
1
KUDOS
Expert's post
I solved it using algebra, for concepts sake, is it possible to use alligation or the scale method here?

In this post above: bob-just-filled-his-car-s-gas-tank-with-20-gallons-of-45790-20.html#p1253824
I 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

Kudos [?]: 18128 [1], given: 236

Intern
Joined: 22 Jul 2014
Posts: 10

Kudos [?]: 9 [0], given: 0

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

### Show Tags

21 Aug 2015, 12:32
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 !

Kudos [?]: 9 [0], given: 0

Current Student
Joined: 02 Jun 2015
Posts: 12

Kudos [?]: 12 [0], given: 41

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

### Show Tags

21 Aug 2015, 18:49
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.

Kudos [?]: 12 [0], given: 41

Intern
Joined: 15 Sep 2012
Posts: 4

Kudos [?]: 5 [0], given: 56

Location: India
Concentration: Operations, Technology
GMAT 1: 500 Q41 V19
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, 20:22
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

E:G
5%:95% >> 1:19

19 is 90% of 190/9. 10% of 190/9 is 19/9.
so we need : 19/9-9/9 = 10/9 gallons more.

Kudos [?]: 5 [0], given: 56

Current Student
Joined: 18 Oct 2014
Posts: 903

Kudos [?]: 454 [0], given: 69

Location: United States
GMAT 1: 660 Q49 V31
GPA: 3.98
Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

### Show Tags

24 May 2016, 05:34
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
x-x/10= 1
x= 10/9

_________________

I welcome critical analysis of my post!! That will help me reach 700+

Kudos [?]: 454 [0], given: 69

Manager
Joined: 23 Dec 2013
Posts: 235

Kudos [?]: 16 [0], given: 21

Location: United States (CA)
GMAT 1: 760 Q49 V44
GPA: 3.76
Re: Bob just filled his car's gas tank with 20 gallons of [#permalink]

### Show Tags

23 May 2017, 20:01
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 per-se. 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.

Kudos [?]: 16 [0], given: 21

Target Test Prep Representative
Affiliations: Target Test Prep
Joined: 04 Mar 2011
Posts: 1802

Kudos [?]: 981 [0], given: 5

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

### Show Tags

26 May 2017, 10:47
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 20-gallon 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

_________________

Jeffery Miller

GMAT Quant Self-Study Course
500+ lessons 3000+ practice problems 800+ HD solutions

Kudos [?]: 981 [0], given: 5

Intern
Joined: 22 Dec 2016
Posts: 6

Kudos [?]: [0], given: 10

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

### Show Tags

21 Jul 2017, 20:05
Besides some easy methods we can also do this question with the help of allegation approach:-
Initially, there was 5% ethanol and we have to add pure ethanol i.e 100% ethanol and the final concentration we need to obtain is 10%.
5%-------------------------100%
------------10%----------
90%-------------------------5% (Use allegation method)

so we have final ratio 90:5 or 18:1
We know we have 5% ethanol 20ml so the amount of 100% ethanol required assume y is:- [18/20 = 1/y] => y = 10/9 (Simple ratios)

Kudos [?]: [0], given: 10

Re: Bob just filled his car's gas tank with 20 gallons of   [#permalink] 21 Jul 2017, 20:05

Go to page   Previous    1   2   3    Next  [ 41 posts ]

Display posts from previous: Sort by