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# 10 machines work at constant rate and working together, they

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Joined: 05 Feb 2006
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10 machines work at constant rate and working together, they [#permalink]  04 Mar 2006, 13:10
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10 machines work at constant rate and working together, they complete a job in 16 hrs. How mny hrs needed if only 8 machines work together to complete a job?
18
20
22
24
26
Manager
Joined: 13 Dec 2005
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10 machine can complete in 16 hrs
1 machine can complete in 16*10 =160 hrs
8 machine can complete in 160/8 =20 hrs

hence Ans B ..20
Intern
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Logic? [#permalink]  05 Mar 2006, 12:48
This may sound wierd. But I want to understand the logic.
10 machines 16hrs
1 machine ?
That is 16/10=1.6hrs. I know it should be 16 *10 but why different way?
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Each machine work at a rate 1/x job in 1 hour
10 machines work at a rate 10/x job in 1 hour

Completed job in 16 hours, so x/10 = 16, x = 160

If there are 8 machines, the job is completed at a rate of 1/20 job per hour, and to finish the job will need 20 hours.

Ans B
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Re: PS machine gmat prep [#permalink]  07 Mar 2006, 08:18
bh10503 wrote:
10 machines work at constant rate and working together, they complete a job in 16 hrs. How mny hrs needed if only 8 machines work together to complete a job?
18
20
22
24
26

My take is B (20). The explaination as follows.
The way I understand this type of problems is by translating the problem into a ratio. In this problem as the number of machine increases, the number of hours required decreases and vice a versa. So the ratio has to be inversly proportional as follows
10/8 = x/16
So 8x = 160
and x = 20

I hope this makes sense.
_________________

Thanks

Manager
Joined: 24 Oct 2005
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Got B as well

10 machine completes in 16*10 =160 hrs
8 machine can complete in 160/8 =20 hrs
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# 10 machines work at constant rate and working together, they

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