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A professional janitor can clean a certain high school in 8 hours, wor

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A professional janitor can clean a certain high school in 8 hours, wor [#permalink]

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Question Stats:

69% (01:46) correct 31% (02:03) wrong based on 182 sessions

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A professional janitor can clean a certain high school in 8 hours, working at a constant rate. A student sentenced to detention can clean that same high school in 20 hours, also working at a constant rate. If the student is paid $7 total per hour and the janitor is paid $21 per hour, how much more would it cost the school to pay the janitor to do the job himself than it would to pay the student and the janitor to do the job together?

A. -$56
B. -$6
C. $0
D. $6
E. $8
[Reveal] Spoiler: OA

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Re: A professional janitor can clean a certain high school in 8 hours, wor [#permalink]

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shasadou wrote:
A professional janitor can clean a certain high school in 8 hours, working at a constant rate. A student sentenced to detention can clean that same high school in 20 hours, also working at a constant rate. If the student is paid $7 total per hour and the janitor is paid $21 per hour, how much more would it cost the school to pay the janitor to do the job himself than it would to pay the student and the janitor to do the job together?

A. -$56
B. -$6
C. $0
D. $6
E. $8


For work questions, there are two useful rules:

Rule #1: If a person can complete an entire job in k hours, then in one hour, the person can complete 1/k of the job
Example: If it takes Sue 5 hours to complete a job, then in one hour, she can complete 1/5 of the job. In other words, her work rate is 1/5 of the job per hour

Rule #2: If a person completes a/b of the job in one hour, then it will take b/a hours to complete the entire job
Example: If Sam can complete 1/8 of the job in one hour, then it will take him 8/1 hours to complete the job.
Likewise, if Joe can complete 2/3 of the job in one hour, then it will take him 3/2 hours to complete the job.

Let’s use these rules to solve the question. . . .
A professional janitor can clean a certain high school in 8 hours
So (applying rule #1), the janitor can clean 1/8 of the school in ONE HOUR

A student sentenced to detention can clean that same high school in 20 hours
So (applying rule #1), the student can clean 1/20 of the school in ONE HOUR

So, COMBINED, the student and janitor can clean (1/8 + 1/20) of the school in ONE HOUR
1/8 + 1/20 = 5/40 + 2/40 = 7/40
So, in ONE HOUR they can clean 7/40 of the school.

Applying rule #2, it will take them 40/7 hours to clean the ENTIRE school.

The janitor earns $21/hour and the student earns $7/hour, so their combined rate is $28/hour.
Their combined wages = (pay rate)(time) = ( $28/hour)(40/7 hours) = $160

Working ALONE, the janitor takes 8 hours and earns $21/hour
So, working alone, the janitor's earnings = (pay rate)(time) = ($21/hour)(8 hours) = $168

$168 - $160 = $8, so the answer is E

Here's a free video on solving work problems: http://www.gmatprepnow.com/module/gmat- ... /video/917
Afterwards, here's a practice question to reinforce your learning: http://www.gmatprepnow.com/module/gmat- ... /video/918



Cheers,
Brent
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Re: A professional janitor can clean a certain high school in 8 hours, wor [#permalink]

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GMATPrepNow

Hi Brent.
Why shouldn't I consider the whole hour for the wage of the janitor plus the student, since they are paid hourly?

Total cost to pay the student and the janitor to do the job together would be 42/7 (in order to be an integer) *($28/hour)=$168

Thanks

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Re: A professional janitor can clean a certain high school in 8 hours, wor [#permalink]

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New post 20 Sep 2016, 05:10
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mmss wrote:
GMATPrepNow

Hi Brent.
Why shouldn't I consider the whole hour for the wage of the janitor plus the student, since they are paid hourly?

Total cost to pay the student and the janitor to do the job together would be 42/7 (in order to be an integer) *($28/hour)=$168

Thanks


There's nothing in the question to suggest that the workers must be paid in 1-hour increments.
So, for example, if you work for 0.5 hours, then you receive half of your hourly rate.

Cheers,
Brent
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Re: A professional janitor can clean a certain high school in 8 hours, wor [#permalink]

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Re: A professional janitor can clean a certain high school in 8 hours, wor   [#permalink] 30 Oct 2017, 18:54
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