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Four carpenters can individually complete a particular task

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Four carpenters can individually complete a particular task  [#permalink]

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New post 05 Jun 2012, 10:02
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A
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D
E

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  25% (medium)

Question Stats:

80% (02:20) correct 20% (03:05) wrong based on 401 sessions

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Four carpenters can individually complete a particular task in 3, 4, 5, and 7 hours, respectively. What is the maximum fraction of the task that can be completed in forty-five minutes if three of the carpenters work together at their respective rates ?

A. 11/15
B. 3/5
C. 11/30
D. 47/80
E. 5/9
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Re: Four carpenters can individually complete a particular task  [#permalink]

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New post 05 Jun 2012, 10:21
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shradhagrover18 wrote:
Four carpenters can individually complete a particular task in 3, 4, 5, and 7 hours, respectively. What is the maximum fraction of the task that can be completed in forty-five minutes if three of the carpenters work together at their respective rates ?

A. 11/15
B. 3/5
C. 11/30
D. 47/80
E. 5/9


In order three of the carpenters to complete the maximum fraction of the task we should select three fastest carpenters. So, those three who complete the task in 3, 4, and 5 hours.

In 1 hour they will complete \(\frac{1}{3}+\frac{1}{4}+\frac{1}{5}=\frac{47}{60}\)th of the task. So, in \(\frac{3}{4}\) hour (45 minutes) they will complete \(\frac{47}{60}*\frac{3}{4}=\frac{47}{80}\)th of the job.

Answer: D.

Hope it's clear.
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Re: Four carpenters can individually complete a particular task  [#permalink]

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New post 05 Jun 2012, 15:33
1/3 + 1/4 + 1/5 = 47/60
47/60 * 3/4 = 47 / 80

D
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Re: Four carpenters can individually complete a particular task  [#permalink]

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New post 05 Jun 2012, 18:05
shradhagrover18 wrote:
Four carpenters can individually complete a particular task in 3, 4, 5, and 7 hours, respectively. What is the maximum fraction of the task that can be completed in forty-five minutes if three of the carpenters work together at their respective rates ?

A. 11/15
B. 3/5
C. 11/30
D. 47/80
E. 5/9


Farmer A, B, C, and D

A's rate: 1/3 job/hour
B's rate: 1/4 job/hour
C's rate: 1/5 job/hour
D's rate: 1/7 job/hour

By intuition we know that the three fastest farmers are A B and C.

As such

(1/3 + 1/4 + 1/5)*(3/4) = ?

47/80 or (D)
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Re: Four carpenters can individually complete a particular task  [#permalink]

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New post 14 Jun 2013, 05:27
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Re: Four carpenters can individually complete a particular task  [#permalink]

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New post 20 May 2017, 00:54
I sometimes do these by taking total work as number of units which is LCM of individual times.
LCM of 3,4,5,7 = 420.

Let total work be 420 units.
Carpenter A does in 1 hour = 420/3 = 140 units.
Carpenter B does in 1 hour = 420/4 = 105 units.
Carpenter C does in 1 hour = 420/5 = 84 units.
Carpenter D does in 1 hour = 420/7 = 60 units.

Clearly, 3 most efficient carpenters are A, B, C. So if we want to find out Max fraction of work possible by 3 carpenters, we have to take these 3 carpenters only.
They do a total of (140+105+84) = 329 units per hour

Now, 45 minutes = 3/4 hours
Work done in 1 hour = 329 units
Work done in 3/4 hours = 329 * 3/4 = 987/4

Now this is what fraction of total work = (987/4)/420 = 47/80.
Hence D

(This is a time taking solution for this question no doubt. Method given by others is better. But I am just trying to add some value by giving an alternate solution. This kind of method can be applied to such questions though, sometimes making them easier)
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Re: Four carpenters can individually complete a particular task   [#permalink] 29 Jul 2018, 03:37
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