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[email protected]: Master | Next Rank: 500 Posts; Posts: 117; Joined: Mon Oct 27, 2008 5:08 pm; Thanked: 1 times

by [email protected] » Tue Dec 23, 2008 6:16 pm

Q) 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

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Source: Beat The GMAT — Problem Solving | Tue Dec 23, 2008 6:16 pm

amitabhprasad: Master | Next Rank: 500 Posts; Posts: 259; Joined: Thu Jan 18, 2007 8:30 pm; Thanked: 16 times

by amitabhprasad » Tue Dec 23, 2008 6:21 pm

I am getting "B"
12 days 6 machine
1 machine = 72 days
8 days = 72/8 = 9 machine
delta = 3 machine

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Ossa: GMAT Instructor; Posts: 30; Joined: Sun Oct 26, 2008 5:42 pm; Thanked: 5 times

That's correct

by Ossa » Tue Dec 23, 2008 6:37 pm

Remember that in questions like this you can add the rates, so in the first case, quantity/6r=12 and in the second case, quantity/xr=8. Accordingly x=9, and so the additional number of machines=3.

amitabhprasad wrote:I am getting "B"
12 days 6 machine
1 machine = 72 days
8 days = 72/8 = 9 machine
delta = 3 machine

Ossa Elhadary, PhD, CISA, PMP
Math Specialist
Test Prep New York
maximize your score, minimize your stress

www.testprepny.com
[email protected]

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parallel_chase: Legendary Member; Posts: 1153; Joined: Wed Jun 20, 2007 6:21 amFollowed by: 2 members; Thanked: 146 times

by parallel_chase » Tue Dec 23, 2008 8:42 pm

Here is a simple way to remember things

work = rate * time * no. of machines

let work be 1

1 = rate * 12 * 6

rate = 1/72

everything remains the same except for no. of machines

work = rate * time * no. of machines

1 = 1/72 * 8 * no. of machines

no. of machines = 72/8 = 9

additional machines required = 9-6 = 3

Hope this helps.

No rest for the Wicked....

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