Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 27 Feb 2010
Posts: 76
Location: Denver

Six machines, each working at the same constant rate
[#permalink]
Show Tags
26 Apr 2010, 21:53
Question Stats:
80% (01:24) correct 20% (01:35) wrong based on 940 sessions
HideShow timer Statistics
Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the Job in 8 days? A. 2 B. 3 C. 4 D. 6 E. 8
Official Answer and Stats are available only to registered users. Register/ Login.




Math Expert
Joined: 02 Sep 2009
Posts: 59725

Re: Help! GMATPrep question
[#permalink]
Show Tags
06 Feb 2012, 02:04
Welcome to GMAT Club. Below is a solution for the question. Hope it helps Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?A. 2 B. 3 C. 4 D. 6 E. 8 Let \(x\) be the time needed for 1 machine to complete the job, so rate of one machine is \(\frac{1}{x}\) (rate is the reciprocal of time) > rate of 6 machines would be \(\frac{6}{x}\). As \(job=time*rate\) > \(1=12*\frac{6}{x}\) > \(x=72\) days needed for 1 machine to complete the job. To complete the job in 8 days \(\frac{72}{8}=9\) machines are needed. Difference: 96=3. Answer: B.
_________________




Senior Manager
Joined: 23 Oct 2010
Posts: 318
Location: Azerbaijan
Concentration: Finance

Re: Help! GMATPrep question
[#permalink]
Show Tags
05 Feb 2012, 23:54
6 machines in 12 hours can make 6*12=72 units. x machines in 8 hours can make x*8=72 units x=9 129=3 more machines are needed hope it helps
_________________
Happy are those who dream dreams and are ready to pay the price to make them come true
I am still on all gmat forums. msg me if you want to ask me smth




Manager
Joined: 12 Jan 2010
Posts: 223
Schools: DukeTuck,Kelogg,Darden

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
27 Apr 2010, 06:02
zz0vlb wrote: Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the Job in 8 days? a.2 b.3 c.4 d.6 e.8 Source: GMAT Prep Another solution which is faster is Since each machine works at a constant rate. The time needs to bought down from 12 to 8. So the new time is 2/3 of the original time. Thus to achieve this we need the rate to be 3/2 of original. So 3/2* 6 = 9 So we need 96 = 3 more machines.



Tutor
Joined: 20 Apr 2012
Posts: 96
Location: Ukraine
GMAT 1: 690 Q51 V31 GMAT 2: 730 Q51 V38
WE: Education (Education)

Re: 6 machines each working @ the same constant rate together
[#permalink]
Show Tags
01 May 2013, 01:06
You can solve this problem without any equations. Just to calculate the rate of 1 machine.
The rate of 1 machine is \(\frac{1}{6*12}=\frac{1}{72}\) part per day.
So, 1 machine in 8 days can do \(\frac{8}{72}=\frac{1}{9}\).
So, to finish the job in 8 days we need 9 machines. Therefore 3 additional machines are required.
The answer is B.



Intern
Joined: 28 Mar 2010
Posts: 24

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
19 Jun 2010, 19:52
Actually, this was more simple than I thought. When I first saw this, I started to panic thinking it's a rate question. But then I took a step back, and it was the simplest of calculations.
So here's the deal:
12 days > 6 machines: 8 days > ?
8 days means you definitely need more machines than the 6 and in such case the equation looks like
12 * 6 = 8 * x
Therefore, x = 72 / 8 = 9
So 96=3 more machines



Intern
Status: Tougher times ...
Joined: 04 Nov 2012
Posts: 35
Location: India
WE: General Management (Manufacturing)

Six machines, each working at the same constant rate, together c
[#permalink]
Show Tags
30 Apr 2013, 23:27
njss750 wrote: 6 machines each working @ the same constant rate together can complete a certain job in 12 days. How many additional machines are reqd (each working at same constant rate) to finish the job in 8 days? 2 3 4 6 8
Pl xplain with wkgs 6 x 12 = (6+x) x 8 9 = 6+x x=3



Math Expert
Joined: 02 Sep 2009
Posts: 59725

Six machines, each working at the same constant rate
[#permalink]
Show Tags
27 Apr 2010, 12:13



Manager
Joined: 01 Feb 2010
Posts: 173

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
26 Apr 2010, 22:50
zz0vlb wrote: Six machines, each working at the same constant rate, together can complete a certain job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the Job in 8 days? a.2 b.3 c.4 d.6 e.8 Source: GMAT Prep let each machine work at rate of x days 1/x+1/x+1/x+1/x+1/x+1/x = 1/12 6/x = 1/12 x = 72 now y is number of machines needed to complete job in 8 days so, y/72 = 1/8 y = 9 required = yx = 3 hence b.



Senior Manager
Joined: 13 Aug 2012
Posts: 397
Concentration: Marketing, Finance
GPA: 3.23

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
14 Nov 2012, 00:32
\(Rate of machine = \frac{1}{m}\)
Draw the equation for 6 machines take 12 days: \(\frac{6}{m}=\frac{1}{12}\) \(m=72 days\)
Let's look for the number of machines to work for 8 days: \(\frac{N}{72}=\frac{1}{8}\) \(N=\frac{72}{8}=9 machines\)
Answer: 9  6 machines = 3 machines more
B



Manager
Status: Bunuel's fan!
Joined: 08 Jul 2011
Posts: 125

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
03 May 2012, 09:47
This is how I did it
6 machines in 1hr do 1/12 job in 1 hr, 1 machine do 1/(12*6) job In 8 days 1 machine do 8/(12*6)=1/9
Thus there is a need of 9 machine.
But I forgot to minus and get the difference so still get this question wrong.



EMPOWERgmat Instructor
Status: GMAT Assassin/CoFounder
Affiliations: EMPOWERgmat
Joined: 19 Dec 2014
Posts: 15729
Location: United States (CA)

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
07 Apr 2018, 10:40
Hi All, We're told that 6 machines, working at the same constant rate, can finish a job in 12 days. We're asked for the number of EXTRA machines will you need to finish the job in 8 days? This type of question is easiest to solve if you convert the info into "work units" Here, we start with 6 machines working for 12 days. This equals (6)(12) = 72 machinedays of work to finish the job. If we wanted to finish the job in 8 days, it would still take 72 machinedays of work, so we'd need 72/8 = 9 machines. We already have 6 machines, so we need 3 MORE machines to get the job done in 8 days. Final Answer: GMAT assassins aren't born, they're made, Rich
_________________
Contact Rich at: Rich.C@empowergmat.comThe Course Used By GMAT Club Moderators To Earn 750+ souvik101990 Score: 760 Q50 V42 ★★★★★ ENGRTOMBA2018 Score: 750 Q49 V44 ★★★★★



Target Test Prep Representative
Status: Founder & CEO
Affiliations: Target Test Prep
Joined: 14 Oct 2015
Posts: 8701
Location: United States (CA)

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
25 Jun 2018, 12:12
srkaleem wrote: Six machines, each working at the same constant rate, together can complete a job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?
A. 2 B. 3 C. 4 D. 6 E. 8 We are given that six machines, each working at the same constant rate, together can complete a certain job in 12 days. Since rate = work/time and if we consider work as 1, the rate of the six machines is 1/12. We need to determine how many additional machines, each working at the same constant rate, will be needed to complete the same job in 8 days. In other words we need to determine how many additional machines are needed to work at a rate of 1/8. Since each machine works at the same constant rate, we can use a proportion to first determine the number of machines needed to work at the rate of 1/8. Our proportion is as follows: 6 machines is to a rate of 1/12 as x machines is to a rate of 1/8. 6/(1/12) = x/(1/8) 72 = 8x x = 9 So we need 9 – 6 = 3 more machines. Answer: B
_________________
5star rated online GMAT quant self study course See why Target Test Prep is the top rated GMAT quant course on GMAT Club. Read Our Reviews If you find one of my posts helpful, please take a moment to click on the "Kudos" button.



Senior Manager
Affiliations: SPG
Joined: 15 Nov 2006
Posts: 267

Re: GMAT PREP (PS)
[#permalink]
Show Tags
05 Aug 2011, 04:50
6 machines require 12 days = 72 machine days how many machines to do the job in 8 days = \(\frac{72 machinedays}{8 days} = 9 machines\)
so, 3 more machines.



Senior Manager
Joined: 10 Jul 2013
Posts: 282

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
12 Aug 2013, 12:50
srkaleem wrote: Six machines, each working at the same constant rate, together can complete a job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?
A. 2 B. 3 C. 4 D. 6 E. 8 Here, 6 × 1/X = 1/12 or, X=72 . 2nd case, n × 1/72 = 1/8 or, n = 9 . additional = 96 = 3



SVP
Status: The Best Or Nothing
Joined: 27 Dec 2012
Posts: 1727
Location: India
Concentration: General Management, Technology
WE: Information Technology (Computer Software)

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
26 Aug 2014, 01:57
6 Machines do \(\frac{1}{12}\) work in 1 day
We require \(\frac{1}{8}\) work to be done in 1 day
Multiplication factor\(= \frac{\frac{1}{8}}{\frac{1}{12}} = \frac{1}{8} * 12 = \frac{3}{2}\)
Additional machines required \(= 6 * \frac{3}{2}  6 = 9  6 = 3\)
Answer = B



Intern
Joined: 24 Mar 2011
Posts: 40
Location: India
Concentration: Marketing, Operations
WE: Operations (Telecommunications)

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
12 Aug 2013, 11:52
srkaleem wrote: Six machines, each working at the same constant rate, together can complete a job in 12 days. How many additional machines, each working at the same constant rate, will be needed to complete the job in 8 days?
A. 2 B. 3 C. 4 D. 6 E. 8 Somehow, I found below method to be working for me  6 Machines in 12 Days do 1 Work i.e. in notation form 6M * 12D = 1W, so 1M * 1D = [1/(6*12)] W How many Machines in 8 Days will complete 1 Work? i.e. xM * 8D = [(x*8)/(6*12)] W = 1W Solving for x in [(x*8)/(6*12)] = 1, we get x = 9 i.e. 9 Machines in 8 Days will do 1 Work. So 96=3 Machines more are required. Ans  B



Intern
Joined: 27 Mar 2014
Posts: 5
Concentration: Accounting
GMAT Date: 09042014
GPA: 4

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
25 Aug 2014, 05:37
since machine and work in inversely related. thus it is inverse proportion
6 x 12 = 8 x N (let n be the number of person required.)
N=9
thus the number of additional person is 96 = 3



Intern
Joined: 10 Dec 2014
Posts: 20
GMAT Date: 12302014

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
26 Dec 2014, 15:00
Six Machines:combined rate=6x Time=12 Let Job be J
6x*12=J
Let M be the number of machines required to complete job in 8 days Then Mx*8=J
6x*12=Mx*8
M=9
Additional=96=3
Is this approach correct??



Veritas Prep GMAT Instructor
Joined: 16 Oct 2010
Posts: 9876
Location: Pune, India

Re: Six machines, each working at the same constant rate
[#permalink]
Show Tags
09 Sep 2019, 07:17
adkikani wrote: Hi VeritasKarishma I wish I could send you a gratitude note for your awesome time efficient approach mentioned here Please confirm your two cents: Logic: more the nos of machines > less no of days Start with no of machines in initial stage: 6 Now we need to decide to multiply by 12/8 or 8/12 Since we need more no of machines to do the work in less time from 12 to 8 days , we need an improper fraction hence 6 * (12/8) = 9 no of machines. Additional machines = 9  6 = 3 Thanks for detailed post here tooYes, that's correct!
_________________
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 >




Re: Six machines, each working at the same constant rate
[#permalink]
09 Sep 2019, 07:17



Go to page
1 2
Next
[ 25 posts ]



