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 Your Progress

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

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 500,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

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 thus the answer is C

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

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)
_________________

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)

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)
_________________

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.
_________________

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)

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.
_________________

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

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?

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.

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

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 : 19/9-9/9 = 10/9 gallons more.

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

C is the answer
_________________

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

gmatclubot

Re: Bob just filled his car's gas tank with 20 gallons of
[#permalink]
24 May 2016, 05:34

Happy New Year everyone! Before I get started on this post, and well, restarted on this blog in general, I wanted to mention something. For the past several months...

It’s quickly approaching two years since I last wrote anything on this blog. A lot has happened since then. When I last posted, I had just gotten back from...

Happy 2017! Here is another update, 7 months later. With this pace I might add only one more post before the end of the GSB! However, I promised that...

The words of John O’Donohue ring in my head every time I reflect on the transformative, euphoric, life-changing, demanding, emotional, and great year that 2016 was! The fourth to...