• NEW! FREE Beat The GMAT Quizzes
Hundreds of Questions Highly Detailed Reporting Expert Explanations
• 7 CATs FREE!
If you earn 100 Forum Points

Engage in the Beat The GMAT forums to earn
100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote ## Five drainage pipe, each draining water from a pool... tagged by: AAPL ##### This topic has 4 expert replies and 0 member replies ### Top Member ## Five drainage pipe, each draining water from a pool... Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days? A) 6 B) 9 C) 10 D) 12 E) 15 The OA is C. Is there a strategic approach to this PS question? Can any experts help me, please? ### GMAT/MBA Expert Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 122 members Upvotes: 1153 GMAT Score: 770 AAPL wrote: Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days? A) 6 B) 9 C) 10 D) 12 E) 15 The OA is C. Is there a strategic approach to this PS question? Can any experts help me, please? If 5 pipes drain a pool in 12 days, then to drain a full pool, we require a total of 5*12 = 60 pipe-days. In other words the number of pipes * days must equal 60. 1 pipe could do the job in 60 days, or 2 pipes could do the job in 30 days, etc. If the job is completed in 4 days, we can solve the following: # pipes * 4 = 60 ---> # pipes = 15 If we started with 5 pipes, we'd need 10 more. The answer is C _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

Enroll in a Veritas Prep GMAT class completely for FREE. Wondering if a GMAT course is right for you? Attend the first class session of an actual GMAT course, either in-person or live online, and see for yourself why so many students choose to work with Veritas Prep. Find a class now!

### GMAT/MBA Expert

Elite Legendary Member
Joined
23 Jun 2013
Posted:
10197 messages
Followed by:
496 members
2867
GMAT Score:
800
Hi AAPL,

We're told that 5 drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. We're asked for the number of ADDITIONAL pipes, each draining water at the same constant rate, would be needed to drain the pool in 4 days.

With these types of questions, it's often best to calculate the total amount of 'work' needed to complete the job (which is the exact approach DavidG showcased). With this question, we can also take advantage of the ratios involved (especially since the numbers 4 and 12 'relate' so nicely to one another).

We currently have 5 pipes to drain the pool - and they can complete the job in 12 days. To complete the job in 4 days - which is exactly ONE THIRD of the time that it currently takes the pipes to do the job - we would need to TRIPLE the number of pipes. Tripling 5 would give us (3)(5) = 15 total pipes. Pay careful attention to the question that is asked though: how many ADDITIONAL pipes would be needed? We already have 5 pipes, so 15 - 5 = 10 ADDITIONAL pipes would be needed.

GMAT assassins aren't born, they're made,
Rich

_________________
Contact Rich at Rich.C@empowergmat.com

### GMAT/MBA Expert

GMAT Instructor
Joined
25 May 2010
Posted:
15361 messages
Followed by:
1865 members
13060
GMAT Score:
790
AAPL wrote:
Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days?

A) 6
B) 9
C) 10
D) 12
E) 15
Let the rate for each pipe = 1 liter per day, implying that the rate for 5 pipes = 5 liters per day.
Since the pool is drained by 5 pipes in 12 days, the pool = rt = 5*12 = 60 liters.
To drain the 60-liter pool in 4 days, the required rate = w/t = 60/4 = 15 liters per day.
Since the rate must increase from 5 liters per day to 15 liters per day -- an increase of 10 liters per day -- 10 additional pipes are needed.

The correct answer is C.

_________________
Mitch Hunt
Private Tutor for the GMAT and GRE
GMATGuruNY@gmail.com

If you find one of my posts helpful, please take a moment to click on the "UPVOTE" icon.

Available for tutoring in NYC and long-distance.
Student Review #1
Student Review #2
Student Review #3

Free GMAT Practice Test How can you improve your test score if you don't know your baseline score? Take a free online practice exam. Get started on achieving your dream score today! Sign up now.

### GMAT/MBA Expert

GMAT Instructor
Joined
25 Apr 2015
Posted:
2852 messages
Followed by:
18 members
43
AAPL wrote:
Five drainage pipes, each draining water from a pool at the same constant rate, together can drain a certain pool in 12 days. How many additional pipes, each draining water at the same constant rate, will be needed to drain the pool in 4 days?

A) 6
B) 9
C) 10
D) 12
E) 15

Since 4 is 1/3 of 12, we need 3 times as many pipes. Therefore, we need 15 pipes, or 10 additional pipes to the 5 pipes we already have.

Alternate Solution:

If 5 pipes require 12 days to drain the pool, we see that it takes 5 x 12 = 60 “pipe-days” to empty the pool. (In other words, 1 pipe would take 60 days to empty the pool, or 2 pipes would take 30 days, or 10 pipes would take 6 days, etc.) The key is that the product of the number of pipes and the number of days must equal 60. Therefore, if we want to drain the pool in 4 days, we will need 60/4 = 15 pipes, which is 10 additional pipes.

_________________

Scott Woodbury-Stewart
Founder and CEO
scott@targettestprep.com

See why Target Test Prep is rated 5 out of 5 stars on BEAT the GMAT. Read our reviews

• 5 Day FREE Trial
Study Smarter, Not Harder

Available with Beat the GMAT members only code

• Get 300+ Practice Questions
25 Video lessons and 6 Webinars for FREE

Available with Beat the GMAT members only code

• Award-winning private GMAT tutoring
Register now and save up to $200 Available with Beat the GMAT members only code • Free Veritas GMAT Class Experience Lesson 1 Live Free Available with Beat the GMAT members only code • FREE GMAT Exam Know how you'd score today for$0

Available with Beat the GMAT members only code

• 5-Day Free Trial
5-day free, full-access trial TTP Quant

Available with Beat the GMAT members only code

• 1 Hour Free
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

• Free Practice Test & Review
How would you score if you took the GMAT

Available with Beat the GMAT members only code

• Magoosh
Study with Magoosh GMAT prep

Available with Beat the GMAT members only code

• Free Trial & Practice Exam
BEAT THE GMAT EXCLUSIVE

Available with Beat the GMAT members only code

### Top First Responders*

1 Brent@GMATPrepNow 39 first replies
2 Ian Stewart 37 first replies
3 Jay@ManhattanReview 32 first replies
4 GMATGuruNY 26 first replies
5 Scott@TargetTestPrep 15 first replies
* Only counts replies to topics started in last 30 days
See More Top Beat The GMAT Members

### Most Active Experts

1 Scott@TargetTestPrep

Target Test Prep

199 posts
2 Max@Math Revolution

Math Revolution

90 posts
3 Brent@GMATPrepNow

GMAT Prep Now Teacher

71 posts
4 GMATGuruNY

The Princeton Review Teacher

45 posts
5 Ian Stewart

GMATiX Teacher

43 posts
See More Top Beat The GMAT Experts