i falter in rate problems :( though being so easy

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divya23: Master | Next Rank: 500 Posts; Posts: 113; Joined: Mon Apr 26, 2010 8:58 amFollowed by: 14 members; Thanked: 2 times

by divya23 » Wed Jun 01, 2011 9:55 pm

6 machines,each wrking at same constant rate,together can complete a job in 12 days.How many additional machines,each woeking at the same rate will be needed to comp job in 8 days?
A.2
B.3
C.4
D.6
E.8

[spoiler]OA = C[/spoiler]

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Source: Beat The GMAT — Problem Solving | Wed Jun 01, 2011 9:55 pm

cans: Legendary Member; Posts: 1309; Joined: Mon Apr 04, 2011 5:34 amFollowed by: 123 members; Location: India; Thanked: 310 times; GMAT Score:750

by cans » Wed Jun 01, 2011 10:24 pm

6 machines do work in 12 days
thus it means 1 machine alone can do work in 12*6 = 72 days
to complete work in 8 days, we need 72/8 = 9 machines
Thus additional of 9-6=3 machines.
IMO B

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Frankenstein: Legendary Member; Posts: 1448; Joined: Tue May 17, 2011 9:55 amFollowed by: 53 members; Location: India; Thanked: 375 times

by Frankenstein » Thu Jun 02, 2011 1:01 am

Hi,
B seems to be correct. Can you check OA again.

Cheers!

Things are not what they appear to be... nor are they otherwise

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Ozlemg: Master | Next Rank: 500 Posts; Posts: 407; Joined: Tue Jan 25, 2011 9:19 amFollowed by: 7 members; Thanked: 25 times

by Ozlemg » Thu Jun 02, 2011 1:09 am

B seems the right answer.

12 days --->6 machines
8 days---> x machines
-------------------------

There is an indirect propotion btw the number of machines and the days.SO,
12*6)/ 8 = 9 machines

9-6 : 3 additional machines are required.

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divya23: Master | Next Rank: 500 Posts; Posts: 113; Joined: Mon Apr 26, 2010 8:58 amFollowed by: 14 members; Thanked: 2 times

by divya23 » Thu Jun 02, 2011 1:29 am

ya its 3 sorry ppl

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galaxian: Senior | Next Rank: 100 Posts; Posts: 79; Joined: Mon Jan 17, 2011 4:51 amFollowed by: 5 members; Location: Hyderabad, India; Thanked: 8 times

by galaxian » Thu Jun 02, 2011 9:58 am

A Good way to approach these problems is to Calculate Amount of work done in Man-Days (Machine days)
Like : 9 Man can do in 7 days => 9*7 Man Days of Work
or 6 Machines in 5 Hours => 6*5 Machine Hours of total work has to be done, this makes finding the unknown easier !

divya23 wrote:6 machines,each wrking at same constant rate,together can complete a job in 12 days.How many additional machines,each woeking at the same rate will be needed to comp job in 8 days?
A.2
B.3
C.4
D.6
E.8

[spoiler]OA = C[/spoiler]

Like this post

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