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:

Alcohol liter = 10% of 40 liters = 4 liters Water liter = 90% of 40 liters = 36 liters New ratio of alcohol = 20% of total solution. Water will be 80% of total. Therefore total = 36/0.8 = 45 liters The added alcohol is 5 liters.
_________________

Another way to solve this: 4 + x = 0.2 (40+x), where 4= 40 litres*10% - current number of litres of alcohol x - number of litres of 100% alcohol to add 40+x - is our original solution plus x litres of 100% alcohol that we added since we need 20% solution, we mutiply 40+x by 0.2

Amount of alcohol in 40 L of solution = 4L Say we add xL of alcohol to it, then amount of alcohol = (4 + x)L and amount of solution now becomes = (40 + x)L

The addition of xL of alcohol should increase the %age of alcohol to 20% (4+x)/(40+x) = 0.2

The solution contains 40*0.1=4 L of alcohol. In order to double the concentration of the solution, we can add 4L but doing so we increase slightly total volume. So, our answer will be something a bit larger than 4L. Let's look at our options... 5L is exactly what we need.
_________________

How many liters of pure alcohol must be added to a 40-liter solution that is 10% alcohol by volume in order to double the alcohol proportion?

A. 4 B. 5 C. 10 D. 20 E. 40

For me it helped to think of the question in terms of fractions and then plug in the answer choices find the new one.

So we know we have the fraction of alcohol to the rest of the solution is 10% or 4/40 and we need the new one to be 20% or 1/5, so the only one that works is B because 4+5/ 40+5 = 9/45=1/5. Not much math needed for this one.

Another way to solve this: 4 + x = 0.2 (40+x), where 4= 40 litres*10% - current number of litres of alcohol x - number of litres of 100% alcohol to add 40+x - is our original solution plus x litres of 100% alcohol that we added since we need 20% solution, we mutiply 40+x by 0.2

Thank you and welcome to GMAT Club!
_________________

The solution contains 40*0.1=4 L of alcohol. In order to double the concentration of the solution, we can add 4L but doing so we increase slightly total volume. So, our answer will be something a bit larger than 4L. Let's look at our options... 5L is exactly what we need.

Good ideas to answer this quicker. I noticed A was a trap and skipped it, but then planned to calculate the percentage using the remaining answer choices. I was just lucky that the first one I did was the right answer.

Since only the content of alcohol is changing(10%--->20%),volume of water remains same i.e 36.And now this volume of water as percentage would be 80% of solution.

So to get to 20% its 8 original mixture : 1 new mixture

40 ltrs : 5 ltrs

Ans: 5

Please explain this further. I don't follow.

I will try to explain this to my best.

Step 1, Basically you put the two proportions on each side, 10% vs. 100% (pure alcohol). Step 2, you put the desired ratio (20%) in the middle. Step 3, you subtract the 10% from 20% which gives you 10% (don't worry about signs), and also 100% - 20%. Step 4, after getting 80% and 10%, simplify the ratio. In this case, 8 to 1. Step 5, put an X in front of both 8 and 1. Step 6, solve for X, in this case we solve the 8x since we know that represents the amount in 40 Gallon. So, 8x=40, x=5. Step 7, jump to the other side (1x) and plug in X. 1(5)= 5, answer is B.

Its a nice solution. I solved it in traditional way of 4+x/40+x = 0.2 => x=5

soundofmusic wrote:

My way of doing this was:

Since only the content of alcohol is changing(10%--->20%),volume of water remains same i.e 36.And now this volume of water as percentage would be 80% of solution.

==> 80/100*(New Volume) = 36 ==> New Vol = 45.

Added alcohol amount is 45-40 = 5

_________________

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MGMAT 6 650 (51,31) on 31/8/11 MGMAT 1 670 (48,33) on 04/9/11 MGMAT 2 670 (47,34) on 07/9/11 MGMAT 3 680 (47,35) on 18/9/11 GMAT Prep1 680 ( 50, 31) on 10/11/11