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# Working independently, Susan can perform a certain task in 6

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01 Nov 2005, 07:23
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Working independently, Susan can perform a certain task in 6 hours and Tom can perform it in 7 hours. If Susan and Tom work together on the task for 3 hours, at which point Susan leaves, how many hours will it take Tom to complete the task alone?

A. 1/14
B. 13/14
C. 1/2
D. 29/42
E. 1
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01 Nov 2005, 07:38
susan and tom can do (1/6)+(1/7) of task in 1 hour.
susan and tom can do 3(1/6)+(1/7) = 13/14 of task in 3 hour.

tom can do 1/7 of task in 1 hour.
tom, after leaving by susan, can do 1-(13/14)=1/14 of task in (1/14)/(1/7)=1/2 hour.
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01 Nov 2005, 07:41
confirm it's 1/2 hour. Buddy makes it short and sweet, no need to elaborate
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02 Nov 2005, 07:36
HIMALAYA wrote:
susan and tom can do (1/6)+(1/7) of task in 1 hour.
susan and tom can do 3(1/6)+(1/7) = 13/14 of task in 3 hour.
tom can do 1/7 of task in 1 hour.
tom, after leaving by susan, can do 1-(13/14)=1/14 of task in (1/14)/(1/7)=1/2 hour.

OA is of course C.

HIMALAYA> shouldnt your second line of calculation also include another 3?? ---->susan and tom can do 3(1/6)+3(1/7) = 13/14 of task in 3 hour.
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03 Nov 2005, 20:18
GMATT73 wrote:
HIMALAYA wrote:
susan and tom can do (1/6)+(1/7) of task in 1 hour.
susan and tom can do 3(1/6)+(1/7) = 13/14 of task in 3 hour.
tom can do 1/7 of task in 1 hour.
tom, after leaving by susan, can do 1-(13/14)=1/14 of task in (1/14)/(1/7)=1/2 hour.

OA is of course C.

HIMALAYA> shouldnt your second line of calculation also include another 3?? ---->susan and tom can do 3(1/6)+3(1/7) = 13/14 of task in 3 hour.

yah...thats correct. it is a typo. it should be: 3{(1/6)+(1/7)}=13/14
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04 Nov 2005, 22:48
GMATT73 wrote:
Working independently, Susan can perform a certain task in 6 hours and Tom can perform it in 7 hours. If Susan and Tom work together on the task for 3 hours, at which point Susan leaves, how many hours will it take Tom to complete the task alone?

A. 1/14
B. 13/14
C. 1/2
D. 29/42
E. 1

C for me too.

3 (1/7 + 1/6) = 13/14, 1/14 is left to be done

T = W/R

T = (1/14)/(1/7) = 1/2
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05 Nov 2005, 06:11
yup got C as well

together they do (1/6+1/7) =13/42 of the job in an hour

So in 3 hour they will do 39/42 of the job so there will be 4/42 or 1/14 part left
it takes Tom to do 1/7 of the job in 1 hour, so it will take 1/2 = (1/14*7) hour for him to complete the 1/14 part.
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hey ya......

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