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Bob and Alice can finish a job together in 3 hours.

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Bob and Alice can finish a job together in 3 hours. [#permalink]

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Bob and Alice can finish a job together in 3 hours. If Bob can do the job by himself in 5 hours, what percent of the job does Alice do?
A) 10
B) 20
C) 40
D) 50
E) 60
[Reveal] Spoiler: OA

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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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New post 27 Oct 2011, 16:56
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Lets supposat W is work Done and for moement we say that

W = 1 unit.

While working together --> W = Rate * Time = (1/A + 1/B)*3


...> 1 = 3 ( 1/A+1/B) --------- (1)

While only Bob is working , W = 1 = Rate of Bob & 5 .....> Rate of Bob = 1/B = 1/5 ---- (2)

Plugging 2 in (1) will give us Rate of Alice = 1/A = 2/15

Work Done by Alice = 2/15*3 = 2/5


% of workd done by Alice = 2/5*100 = 40% ---(C)
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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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You can also plug in numbers. For example, Bob and Alice work at a donut factory and make 15 donuts which is the job ( I picked this as a SMART number).

Bob on his own works 15/5 = 3 donuts per hour.

Bob and Alice work 15/5 = 5 donuts per hour so Alice works 2 donuts/hour

To find out the percentage, Alice wroks 2 donuts/hr x 3 hours = 6 donuts per hour.

Therefore 6 donuts/15 donuts = 2/5 = 40%

hope this helped :)
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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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Hi All,

This prompt is a variation on a Work Formula question. It can actually be solved in a variety of ways - based on the information that we're given, here's a method that minimizes the number of calculations that you have to do:

We're told that Bob and Alice can finish a job together in 3 hours and that Bob can do the job by himself in 5 hours. We're asked what PERCENT of the job that Alice completes.

Since Bob can do the job in 5 hours, he completes 1/5 of the job per hour. We're told that he and Alice BOTH work for 3 hours, so Bob completes...

(3 hours)(1/5 of the job per hour) = 3/5 of the job = 60% of the job.

Since Bob completes 60% of the job, Alice must complete the other 40%.

Final Answer:
[Reveal] Spoiler:
C


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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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New post 29 Mar 2017, 12:45
Hello from the GMAT Club BumpBot!

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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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New post 29 Mar 2017, 12:53
1/5 + 1/x = 1/3
1/x = 1/3 - 1/5
1/x = 2/5

2/5 x 100 = 40% Answer is C
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Re: Bob and Alice can finish a job together in 3 hours. [#permalink]

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386390 wrote:
Bob and Alice can finish a job together in 3 hours. If Bob can do the job by himself in 5 hours, what percent of the job does Alice do?
A) 10
B) 20
C) 40
D) 50
E) 60


Since Bob and Alice can complete a job in 3 hours, the combined rate of Bob and Alice = 1/3. Since Bob can do the job by himself in 5 hours, his rate is 1/5.

Thus, the rate of Alice is 1/3 - 1/5 = 5/15 - 3/15 = 2/15.

Thus, when working together, the combined work is 1/3 and Alice's work is 2/15.

Since Alice has worked for 3 hours, her work is 3 x 2/15 = 2/5. Furthermore, since we consider the complete job the number 1, the percentage of the job done by Alice is:

(2/5)/1 = 2/5 = 40%

Answer: C
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Re: Bob and Alice can finish a job together in 3 hours.   [#permalink] 04 Apr 2017, 15:01
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