Five drainage pipes, each draining water from a pool at the

Question

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

I have got the right answer as 10. But I need to make sure that my method is correct.

Total days taken by 5 pipes = 12*5 = 60 pipe days

Pipes taken to drain the pool in 4 days = 60/4 = 15. Extra pipes = 15-5 = 10.

Bunuel · Answer

Yes, your approach is correct.

One can also do: since  rate*time=job , then in order to decrease the time needed to do the job 3 times we need to increase rate per day 3 times to do the same job, so we need 5*3=15 pipes. Difference 15-5=10.

Answer: C.

PareshGmat · Answer

This is an inverse proportional problem......
5 pipes in 12 days; So for 4 days, it will be = 12 x 5 / 4 = 15
So, 15-5 = 10

madzstar · Answer

i really suck at these

unceldolan · Answer

The pipes now (5) need 12 days to drain the pool. Now we want to drain the pool in 1/3 of the time. Hence we need 3 times as many pipes (15). Since we've already got 5 pipes, we need  10 additional pipes . 
Silly task, you have to look exactly what they want...almost made the mistake!

gracie · Answer

r=rate of each pipe
x=total pipes needed
(12)(5r)=(4)(xr)
x=15
15-5=10 new pipes

ScottTargetTestPrep · Answer

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

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

Five drainage pipes, each draining water from a pool at the

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

Five drainage pipes, each draining water from a pool at the

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards