M25 #16 : GMAT Club Tests

M25 #16

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Status: Looking for High GMAT Score

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Re: M25 #16 [#permalink]

04 Jul 2012, 04:06

Jack - 8 hrs - in 1 hr - 1/8 Tom - 12 hrs - in 1 hr- 1/12
Jack works for 4 hours so he does 1/8/4 = 1/2 and 1/2 is left
In one hour work done by Jack and tom : ab/a+b = 8*12/8+12 = 24/5 = 4.8
In one hour - 24/5 so in how many hour 1/2 = 24/5*1/2 = 24/10 = 2.4 hrs
so earlier 4 + 2.4 hrs = 6.4 hrs Answer D
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Re: M25 #16 [#permalink]

31 Aug 2012, 20:36

This question took me 2mins 2 secs which I feel is on the higher side. I need to work further on reducing the time taken because I knew the approach but just kind of missed a trick at the last point of the solution.

I followed a similar approach as per the work formula. My solution goes as follows --:

Jack : 8 hours to complete the job. Hence in 4 hours he will complete 1/2 of the job.

At this point Tom (who takes 12 hours for the job) joins in.

Together, Jack and Tom complete (1/8+1/12) of the job is 1 hour i.e. 5/24 of the job in 1 hour.

Here, I missed a simple trick i.e. 5/24 in 1 hour hence 24/5 for the entire job i.e. (24/5)/2 for half the job. This could have reduced my time about 10-15 secs.

I spent relatively longer time on this step. Since we need time for 1/2 of the job i.e. 12/24 it can be split as 5/24+5/24+2/24 i.e. 1 hour + 1 Hour + <~half hour.

Hence total time needed by Jack = 4 hours + ~2.5 Hours = greater than 6 hours but less than 6.5 hours. D is the only option hence I chose D.
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Re: M25 #16 [#permalink] 31 Aug 2012, 20:36

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M25 #16

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