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A mathematics professor can grade 20 exam papers in one day. How long

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A mathematics professor can grade 20 exam papers in one day. How long [#permalink]

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New post 20 Mar 2017, 03:41
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A
B
C
D
E

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Question Stats:

82% (00:43) correct 18% (01:15) wrong based on 61 sessions

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A mathematics professor can grade 20 exam papers in one day. How long will it take him to grade 30 papers if he is joined a colleague grading twice as fast?

A. 1/2 day
B. 3/4 day
C. 1 day
D. 3/2 days
E. 2 days
[Reveal] Spoiler: OA

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A mathematics professor can grade 20 exam papers in one day. How long [#permalink]

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New post 20 Mar 2017, 04:17
Bunuel wrote:
A mathematics professor can grade 20 exam papers in one day. How long will it take him to grade 30 papers if he is joined a colleague grading twice as fast?

A. 1/2 day
B. 3/4 day
C. 1 day
D. 3/2 days
E. 2 days


professor can grade 20 exam papers in one day which means 10 exam papers in half day
colleague can grade 40 exam papers in one day which means 20 exam papers in half day

so both of them working together will finish grading 30 exam papers in half day

Hence option A is correct
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Re: A mathematics professor can grade 20 exam papers in one day. How long [#permalink]

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Bunuel wrote:
A mathematics professor can grade 20 exam papers in one day. How long will it take him to grade 30 papers if he is joined a colleague grading twice as fast?

A. 1/2 day
B. 3/4 day
C. 1 day
D. 3/2 days
E. 2 days


We are given that the rate of a math professor is 20 papers per day. If he were to join with his colleague, who grades twice as fast, or 40 papers per day, the combined rate of the 2 professors would be 20 + 40 = 60 papers per day.

Thus, it would take 30/60 = 1/2 a day to grade 30 papers.

Answer: A
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A mathematics professor can grade 20 exam papers in one day. How long [#permalink]

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New post 22 Mar 2017, 09:56
Bunuel wrote:
A mathematics professor can grade 20 exam papers in one day. How long will it take him to grade 30 papers if he is joined a colleague grading twice as fast?

A. 1/2 day
B. 3/4 day
C. 1 day
D. 3/2 days
E. 2 days


Efficiency of 1st professor = 20 papers/day
Efficiency of 2nd professor = 40 papers/day

Combined efficiency of 1st & 2nd Professor is 60 papers/day

Quote:
How long will it take him to grade 30 papers


Time taken by the 2 professors to grade 30 papers is \(\frac{30}{60}\) = \(\frac{1}{2}\) days..

Thus, the correct , answer must be (A) \(\frac{1}{2}\) days...
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Re: A mathematics professor can grade 20 exam papers in one day. How long [#permalink]

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New post 23 Mar 2017, 22:40
individually they have 20 and 40 papers in a days
together they can have 60 papers in 1 days ... do for 30 it required 1/2 day ...
A is the answer
Re: A mathematics professor can grade 20 exam papers in one day. How long   [#permalink] 23 Mar 2017, 22:40
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