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AAPL: Moderator; Posts: 2599; Joined: Sun Oct 29, 2017 2:08 pm; Followed by:2 members

Five drainage pipe, each draining water from a pool...

by AAPL » Fri Feb 23, 2018 7:27 am

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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?

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Source: Beat The GMAT — Problem Solving | Fri Feb 23, 2018 7:27 am

DavidG@VeritasPrep: Legendary Member; Posts: 2663; Joined: Wed Jan 14, 2015 8:25 am; Location: Boston, MA; Thanked: 1153 times; Followed by:128 members; GMAT Score:770

by DavidG@VeritasPrep » Fri Feb 23, 2018 9:31 am

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

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by [email protected] » Fri Feb 23, 2018 11:25 am

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.

Final Answer: C

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Rich

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by GMATGuruNY » Fri Feb 23, 2018 12:46 pm

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.

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by Scott@TargetTestPrep » Fri Jun 14, 2019 2:46 pm

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.

Answer: C

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