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Machine A working alone can complete a job in 3 1/2 hours

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Machine A working alone can complete a job in 3 1/2 hours [#permalink] New post 08 Mar 2011, 01:23
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Question Stats:

81% (01:54) correct 19% (01:19) wrong based on 67 sessions
Machine A working alone can complete a job in 3 1/2 hours. Machine B working alone can do the same job in 4 2/3 hours. How long will it take both machines working together at their respective constant rates to complete the job?

A. 1 hr 10 min
B. 2hr
C. 4hr 5 min
D. 7hr
E. 8 hr 10 min
[Reveal] Spoiler: OA
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Re: p/s work [#permalink] New post 08 Mar 2011, 01:37
Lolaergasheva wrote:
Machine A working alone can complete a job in hours. Machine B working alone can do the same job in hours. How long will it take both machines working together at their respective constant rates to complete the job?

(A) 1 hr 10 min
(B) 2 hr
(C) 4 hr 5 min
(D) 7 hr
(E) 8 hr 10 min

I got answer E but it is incorrect


I think the question is incomplete in its present form. The details of number of hours for both machines is not there. If they were given (say x and y), then you could get the total number of hours by simple formula of 1/x+1/y = 1/combined rate
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Re: p/s work [#permalink] New post 08 Mar 2011, 02:16
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Lolaergasheva wrote:
Machine A working alone can complete a job in hours. Machine B working alone can do the same job in hours. How long will it take both machines working together at their respective constant rates to complete the job?

(A) 1 hr 10 min
(B) 2 hr
(C) 4 hr 5 min
(D) 7 hr
(E) 8 hr 10 min

I got answer E but it is incorrect


PLEASE CHECK THE QUESTIONS WHEN POSTING.

Original question:
Machine A working alone can complete a job in 3 1/2 hours. Machine B working alone can do the same job in 4 2/3 hours. How long will it take both machines working together at their respective constant rates to complete the job?
A. 1 hr 10 min
B. 2hr
C. 4hr 5 min
D. 7hr
E. 8 hr 10 min

General formula for multiple entities is \frac{1}{t_1}+\frac{1}{t_2}+\frac{1}{t_3}+...+\frac{1}{t_n}=\frac{1}{T}, where T is time needed for these entities to complete a given job working simultaneously.

For example if:
Time needed for A to complete the job is A hours;
Time needed for B to complete the job is B hours;
Time needed for C to complete the job is C hours;
...
Time needed for N to complete the job is N hours;

Then: \frac{1}{A}+\frac{1}{B}+\frac{1}{C}+...+\frac{1}{N}=\frac{1}{T}, where T is the time needed for A, B, C, ..., and N to complete the job working simultaneously.

For two and three entities (workers, pumps, ...):

General formula for calculating the time needed for two entities A and B working simultaneously to complete one job:

Given that t_1 and t_2 are the respective individual times needed for A and B (pumps, ...) to complete the job, then time needed for A and B working simultaneously to complete the job equals to T_{(A&B)}=\frac{t_1*t_2}{t_1+t_2} hours, which is reciprocal of the sum of their respective rates (\frac{1}{t_1}+\frac{1}{t_2}=\frac{1}{T}).

For our original questions it'll be: 1/(7/2)+1/(14/3)=1/T --> T=2.

Answer: B.

Must know to solve work problems: word-translations-rates-work-104208.html#p812628
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Re: Machine A working alone can complete a job in 3 1/2 hours [#permalink] New post 07 Aug 2014, 09:18
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Re: Machine A working alone can complete a job in 3 1/2 hours   [#permalink] 07 Aug 2014, 09:18
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Machine A working alone can complete a job in 3 1/2 hours

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