GMAT Question of the Day - Daily to your Mailbox; hard ones only

It is currently 06 Dec 2019, 10:41

Close

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
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.

Close

Request Expert Reply

Confirm Cancel

How much water should be added to 10 liters of a 20%-solution of alcoh

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:

Hide Tags

Find Similar Topics 
Director
Director
avatar
Joined: 22 Nov 2007
Posts: 854
How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post Updated on: 22 Jul 2015, 10:24
3
35
00:00
A
B
C
D
E

Difficulty:

  35% (medium)

Question Stats:

71% (02:06) correct 29% (02:30) wrong based on 378 sessions

HideShow timer Statistics

How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters

Originally posted by marcodonzelli on 02 Mar 2008, 09:04.
Last edited by Bunuel on 22 Jul 2015, 10:24, edited 1 time in total.
Renamed the topic, edited the question, added the OA and moved to PS forum.
Most Helpful Expert Reply
Veritas Prep GMAT Instructor
User avatar
V
Joined: 16 Oct 2010
Posts: 9850
Location: Pune, India
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 18 Sep 2017, 22:28
5
1
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


Responding to a pm:

Right now, we have a 20% solution of alcohol.
We need to reduce the concentration of alcohol by 75% i.e. reduce it by 15% to make it 5%.

So now, you have a 10 lt solution of 20% alcohol and you want to make it 5% alcohol by adding pure water (i.e. 0% alcohol)

w1/w2 = (A2 - Aavg)/(Aavg - A1) = (0 - 5)/(5 - 20) = 1/3

So for every 1 lt of original solution, you need 3 lt of water. So for 10 litres of original solution, you need 30 lt of water.

Answer (C)
_________________
Karishma
Veritas Prep GMAT Instructor

Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >
General Discussion
Director
Director
avatar
Joined: 10 Sep 2007
Posts: 654
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 02 Mar 2008, 10:51
2
2
In original Content of 10 litre, there is 2 litre alcohol and 8 litre water.

Question ask to reduce concentration of alcohol to 5% (reduce concentration from 20% by 75%).

So lets assume X litre water is added.

So alcohol remains 2, water is 8+X and total content is 10+X.

Now 2*100/(10+X) = 5 => X = 30

Answer C.
Director
Director
avatar
Joined: 22 Nov 2007
Posts: 854
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 02 Mar 2008, 10:58
1
abhijit_sen wrote:
In original Content of 10 litre, there is 2 litre alcohol and 8 litre water.

Question ask to reduce concentration of alcohol to 5% (reduce concentration from 20% by 75%).

So lets assume X litre water is added.

So alcohol remains 2, water is 8+X and total content is 10+X.

Now 2*100/(10+X) = 5 => X = 30

Answer C.


very quickly: 2/(10+x)=1/20, x=30
Manager
Manager
avatar
B
Joined: 10 Mar 2014
Posts: 180
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 22 Jul 2015, 09:34
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?
 25 liters
 27 liters
 30 liters
 32 liters
 35 liters


Hi Bunuel,

Could you please clarify 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75%?. I am getting confusion in this term.

Regards
Math Expert
User avatar
V
Joined: 02 Sep 2009
Posts: 59587
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 22 Jul 2015, 10:27
1
1
PathFinder007 wrote:
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?
 25 liters
 27 liters
 30 liters
 32 liters
 35 liters


Hi Bunuel,

Could you please clarify 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75%?. I am getting confusion in this term.

Regards


The question asks about the amount of water we should add to 10 liters of a 20%-solution of alcohol for the solution to become 5%-solution of alcohol (20% reduced by 75% is 5%).
_________________
Intern
Intern
avatar
Joined: 02 Mar 2015
Posts: 4
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 22 Jul 2015, 13:08
1
1
The quantity of alcohol in the actual mixture is 2 litres (20% of 10 litres). Now it is said that the concentration is reduced by 75% or 3/4th of 20% which is 5%. Now in the new mixture this 2 litres alcohol is 5% of the total mixture therefore the total mixture would be 2 *20=40 litres. So 30 litres is added. (Note 5% is 1/20)
Intern
Intern
avatar
Joined: 11 Dec 2013
Posts: 7
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 26 Jul 2015, 22:46
sudipnayak93 wrote:
The quantity of alcohol in the actual mixture is 2 litres (20% of 10 litres). Now it is said that the concentration is reduced by 75% or 3/4th of 20% which is 5%. Now in the new mixture this 2 litres alcohol is 5% of the total mixture therefore the total mixture would be 2 *20=40 litres. So 30 litres is added. (Note 5% is 1/20)


10 litres is containing 20% alcoholol...means 2 litres of alcohol and 8 litres of water.
Now the alchol percentage is to reduced by 75%..that means 75% of 20%...that means in the new solution, the content of alcohol should be 5%. (Reduction of 15%).
So that means, the 2 litres of alcohol will make 5% of new solution..
5%-->2 litres
100%-->40 litres..
That means an additional 30 litres (10 litres was initial solution)...and this additional 30 litres is only water.
To cross check--> in new solution..40 litres , 2 litres is alcohol and 38 litres is water.
2 litres = 5% and 38 litres=95%.

Regards
Naresh
Director
Director
User avatar
D
Joined: 05 Mar 2015
Posts: 980
Reviews Badge
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 02 May 2016, 10:12
2
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


Let x ltr water to be added
2ltr alcohol to be represented as (20(1-3/4(new soln.=10+x)))

2=5%*(10+x)-------->x=30
Ans C
Senior Manager
Senior Manager
User avatar
B
Joined: 28 Jun 2015
Posts: 279
Concentration: Finance
GPA: 3.5
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 26 Jul 2017, 23:05
3
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


Originally, the solution had 2L of alcohol and 8L of water. To reduce the concentration of alcohol by 75% means to bring down the alcohol level to 2L * 25% = 0.5L, so that the solution would contain 9.5L water and 0.5L alcohol.

\(\frac{8+x}{10+x} = \frac{95}{100}\)

\(800 + 100x = 950 + 95x\)

\(5x = 150\)

\(x = 30\). Ans - C.
_________________
I used to think the brain was the most important organ. Then I thought, look what’s telling me that.
VP
VP
avatar
P
Joined: 07 Dec 2014
Posts: 1218
How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post Updated on: 19 Sep 2019, 20:08
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


let w=amount of water to be added
.8*10+w=[1-(.2*.25)](10+w)→
w=30 liters
C

Originally posted by gracie on 27 Jul 2017, 11:21.
Last edited by gracie on 19 Sep 2019, 20:08, edited 1 time in total.
Senior SC Moderator
avatar
V
Joined: 22 May 2016
Posts: 3723
How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 30 Jul 2017, 12:55
2
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters

One iteration of a weighted average equation here is
(Concentration of A)(Vol A) + (Concen of B)(Vol B) = (Desired Concen)(Total vol = A+B)

This question is a bit unusual.

First, often, the desired concentration is given. Second, it might be hard to discern which value to reduce by 75 percent. In the equation above for this question,

A = 10 liters of soltion with 20% (=.20) alcohol
B = x liters of water with 0% (=0.0) alcohol

If water is zero percent alcohol, its percentage can't be reduced any further. The question therefore asks that 20 percent concentration be reduced by 75 percent:

(.20)(.25) = .05. The latter is the desired concentration of resultant solution.

In the equation, you don't have to insert second term on LHS. Doing so just makes clear that you are adding x, but its concentration of alcohol has no weight. So

(.20)10 + (0)x = (.05)(10 + x)

2 + 0x = .5 + .05x

1.5 = .05x

\(\frac{1.5}{.05}\)= x

x = 30

Answer C
_________________
SC Butler has resumed! Get two SC questions to practice, whose links you can find by date, here.

Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has -- Margaret Mead
Manager
Manager
User avatar
S
Joined: 30 May 2012
Posts: 197
Location: United States (TX)
Concentration: Finance, Marketing
GPA: 3.3
WE: Information Technology (Consulting)
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 28 Sep 2017, 09:31
How does one solve this using allegation approach? Bunuel, Abhishek009, VeritasPrepKarishma anyone?
Intern
Intern
avatar
B
Joined: 08 Sep 2011
Posts: 1
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 05 Apr 2019, 13:55
Hi, why does this approach not work
20% of 10 litres of solution + x litres of water = 5% of (10+x) litres of solution

x being the added quantity of water
Intern
Intern
avatar
B
Joined: 28 Aug 2016
Posts: 11
Reviews Badge
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 18 Apr 2019, 19:13
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


We can back-solve as well:

20% of 10 liters is 2 liters.
20% reduced by 75% is 5%.

If we check C -> 30 + 10 (initial liters) = 40 liters, 2/40 = 1/20 or 5%
Veritas Prep GMAT Instructor
User avatar
V
Joined: 16 Oct 2010
Posts: 9850
Location: Pune, India
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 18 Apr 2019, 20:30
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


Concentration of alcohol = 20% = C1
A reduction by 75% means the concentration of alcohol should become 5% = Cavg
Concentration of alcohol in water = 0% = C2

10/w2 = (0 - 5) / (5 - 20) = 1/3

w2 = 30 lts

Answer (C)

Check out the weighted avg blog to understand this:
https://www.veritasprep.com/blog/2011/0 ... -averages/
https://www.veritasprep.com/blog/2011/0 ... -mixtures/
_________________
Karishma
Veritas Prep GMAT Instructor

Learn more about how Veritas Prep can help you achieve a great GMAT score by checking out their GMAT Prep Options >
Intern
Intern
avatar
B
Joined: 23 Aug 2018
Posts: 2
Concentration: Economics, Organizational Behavior
GMAT 1: 600 Q36 V31
Premium Member CAT Tests
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 19 Sep 2019, 16:52
Trying to be consistent and use a table to solve all mixture problems. Having some trouble....Bunuel could you critique me on where i am going wrong:

Amount(L) Soln
20% (alcohol) 10 20(10)
100%(water) x 100x
5% (alcohol) 10 + x 5(10 + x)

20(10) + 100x = 5(10 +x)
200 - 50 +100x = 5x
150 = -95x
Answer = some negative....

As mentioned above i see logically it is 150/5 = x........x= 30
Director
Director
User avatar
D
Joined: 05 Mar 2015
Posts: 980
Reviews Badge
How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 19 Sep 2019, 21:13
1
FitnessforFriends wrote:
Trying to be consistent and use a table to solve all mixture problems. Having some trouble....Bunuel could you critique me on where i am going wrong:

Amount(L) Soln
20% (alcohol) 10 20(10)
100%(water) x 100x
5% (alcohol) 10 + x 5(10 + x)

20(10) + 100x = 5(10 +x)
200 - 50 +100x = 5x
150 = -95x
Answer = some negative....

As mentioned above i see logically it is 150/5 = x........x= 30



How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75%

Hi FitnessforFriends
the above highlighted part should be
20(10) = 5(10 +x)

as the amount of alcohol remains same ...u have to calculate % of alcohol in new solution

thus 20(10) = 5(10 +x) ==> 200= 50+5x ==> 150/5=x===>30 :)

hope it helps
Manager
Manager
avatar
S
Joined: 10 Dec 2017
Posts: 150
Location: India
CAT Tests
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 25 Sep 2019, 13:01
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters



Intial= 10*0.20= 2Litres
Initial quantity of alcohol will remain in the solution but it will get diluted. As the concentration of alcohol in the solution is reduced by 75% means 25% of initial quantity of alcohol is present in the new solution which is equal to 2*0.25=1/2 litre=5%(as 2 litres of alcohol= 20%)

Final= (10+x)0.05
Initial quantity of alcohol in the solution=Final quantity of alcohol in the solution
2=(10+x)*0.05
200/5=10+x
40=10+x
X=30 Litres
SVP
SVP
User avatar
D
Joined: 03 Jun 2019
Posts: 1876
Location: India
Premium Member Reviews Badge CAT Tests
Re: How much water should be added to 10 liters of a 20%-solution of alcoh  [#permalink]

Show Tags

New post 25 Sep 2019, 19:04
marcodonzelli wrote:
How much water should be added to 10 liters of a 20%-solution of alcohol to reduce the concentration of alcohol in the solution by 75% ?

A. 25 liters
B. 27 liters
C. 30 liters
D. 32 liters
E. 35 liters


Alcohol = 20%*10 = 2 litres
If alcohol = 5%T= 2
T = 40 litres

Water to be added = 40-10=30 litres

IMO C

Posted from my mobile device
GMAT Club Bot
Re: How much water should be added to 10 liters of a 20%-solution of alcoh   [#permalink] 25 Sep 2019, 19:04
Display posts from previous: Sort by

How much water should be added to 10 liters of a 20%-solution of alcoh

  new topic post reply Question banks Downloads My Bookmarks Reviews Important topics  





Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne