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abhi75: Master | Next Rank: 500 Posts; Posts: 105; Joined: Tue Oct 16, 2007 5:57 am; Thanked: 3 times

by abhi75 » Sun Dec 16, 2007 1:09 pm

Six M/C, each working at the same constant rate, together can complete a certain job in 12 days. How many additional m/c's, each working at the same constant rate, will be needed to complete a job in 8 days.

a) 2 b) 3 c) 4 d) 6 e) 8.

Can anyone please solve this question. I will post the OA soon.

Thanks.

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Source: Beat The GMAT — Problem Solving | Sun Dec 16, 2007 1:09 pm

sankruth: Master | Next Rank: 500 Posts; Posts: 195; Joined: Sun Oct 21, 2007 4:33 am; Thanked: 10 times

by sankruth » Sun Dec 16, 2007 1:16 pm

Assume r = Rate of doing work for a machine

Rate x time = work

6r x 12 = 1 OR r = 1/72

Let number of machines be x
so x(1/72) . 8 = 1

x = 9.

So the number of additional machines = 9-6 = 3 (b)

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abhi75: Master | Next Rank: 500 Posts; Posts: 105; Joined: Tue Oct 16, 2007 5:57 am; Thanked: 3 times

by abhi75 » Sun Dec 16, 2007 1:55 pm

Yes 3 is the correct answer. I am not sure how did you take the equation 6r X 12 = 1. I was not sure about the 6r part.

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sankruth: Master | Next Rank: 500 Posts; Posts: 195; Joined: Sun Oct 21, 2007 4:33 am; Thanked: 10 times

by sankruth » Sun Dec 16, 2007 2:55 pm

Lets look at it another way...

If it takes 12 days to do a job then the rate = 1/12 i.e. 1/12th of a job will be done in a day.

If 6 machines will perform 1/12th of a job in a day, then each machine will do 1/(12 x 6)th job a day

In the equation 6r = 1/12, so r = 1/72

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andes1: Senior | Next Rank: 100 Posts; Posts: 48; Joined: Sun Nov 25, 2007 3:27 pm; Location: Colombia Southamerica; Thanked: 1 times

by andes1 » Tue Dec 18, 2007 6:06 pm

think on it in an easy way

machines....job.....days..................so
6................ 1...... 12............... 6x1x12 = 72
?................ 1....... 8.................?x1x8=....?8

then 72/?8 = ................. ?=9

then 9-6=3(additional machines)
answ= b)

LEARNING ENGLIS H

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camitava: Legendary Member; Posts: 645; Joined: Wed Sep 05, 2007 4:37 amFollowed by: 5 members; Location: India; Thanked: 34 times

by camitava » Wed Dec 19, 2007 1:27 am

Kooooooooooooooolllllllllllllllllll man, andes1! Great approach indeed ...

Correct me If I am wrong

Regards,

Amitava

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