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moneyman: Master | Next Rank: 500 Posts; Posts: 468; Joined: Sat Mar 03, 2007 10:17 pm; Thanked: 5 times

by moneyman » Sun Apr 27, 2008 9:35 am

Six machines,each working at the same constant rate can complete a job in 12 days. How many additional machines,each working at the same constant rate, will be needed to finish the work in 8 days ?

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8

Ans-3

Maxx

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Source: Beat The GMAT — Problem Solving | Sun Apr 27, 2008 9:35 am

freedsl: Junior | Next Rank: 30 Posts; Posts: 25; Joined: Wed Mar 26, 2008 10:30 am; Location: so cal

by freedsl » Sun Apr 27, 2008 10:18 am

if 6 machines do it at the same rate and finish in 12, then find the rate that one machine can do this on its own.

6/m=12, then m=72 days per machine , the rate one machine can do the job.

then 72/8 gives the machines you need to finishin 8 days, which is 9.

the additional machines are 9-6=3.
can someone double check my work? tks

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Sunny22uk: Master | Next Rank: 500 Posts; Posts: 144; Joined: Thu Apr 17, 2008 3:33 pm; Thanked: 8 times

by Sunny22uk » Sun Apr 27, 2008 7:50 pm

3 is the correct answer

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moneyman: Master | Next Rank: 500 Posts; Posts: 468; Joined: Sat Mar 03, 2007 10:17 pm; Thanked: 5 times

by moneyman » Mon Apr 28, 2008 7:46 am

Sorry but I still dint get the catch here!! Can somebody pls explain in an easier way??

Maxx

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AleksandrM: Legendary Member; Posts: 566; Joined: Fri Jan 04, 2008 11:01 am; Location: Philadelphia; Thanked: 31 times; GMAT Score:640

by AleksandrM » Mon Apr 28, 2008 9:49 am

It looks like freesl meant to write m/6 = 12, and m = 72.

If it takes one machine 72 days to finish the job then how many machines will you need in order to finish the job in 8 days?

In other words, to how many machines do you need to distribute the work in order to cut the total work from 72 days to 8 days. Therefore:

72/8 = 9

It takes 9 machines to complete this job in 8 days, compared to 1 machine completing the job in 72 days, or 6 machines completing it in 12 days.

Since you already have 6 machines, you need 3 additional machines to make it 9 machines total.

Think of it this way.

There is a job that needs to be done. The job takes 72 days. To how many machines would you have to distribute this job in order to cut it down from 72 days to 8 days. That is why you have to divide 72 days (the job) by the number of days/hours in which you would like to finish the job.

https://second-lap.blocked/

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aj5105: Legendary Member; Posts: 1169; Joined: Sun Jul 06, 2008 2:34 amFollowed by: 1 member; Thanked: 25 times

by aj5105 » Sun Oct 05, 2008 8:19 am

8*n/ 72 = 12*6/72

n = number of machines taken to complete the task in 8 days.
Rate = 72 days per machine

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