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Time formula - no answer choices

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by SaraLotfy » Wed Oct 02, 2013 9:24 am
At supersonic corporation, the time required for a machine to complete a job is determined by the formula t= Sqrt/W + Sqrt/(W+1) where W= the weight of the machine in pounds and t=the hours required to complete the job. If machine A weighs 8 pounds, and machine B weighs 7 pounds, how many hours will it take the two machines to complete the job together?
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by theCodeToGMAT » Wed Oct 02, 2013 9:30 am
SaraLotfy wrote:At supersonic corporation, the time required for a machine to complete a job is determined by the formula t= Sqrt/W + Sqrt/(W+1) where W= the weight of the machine in pounds and t=the hours required to complete the job. If machine A weighs 8 pounds, and machine B weighs 7 pounds, how many hours will it take the two machines to complete the job together?
Do you mean
t= Sqrt/W + Sqrt/(W+1)
or t = sqrt(W) + sqrt(W+1)
or t = sqrt(1/W) + sqrt(1/(W+1))

Sorry, i couldn't understand what you meant to write...
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by SaraLotfy » Wed Oct 02, 2013 9:32 am
Sorry if it's unclear. The equation is t = sqrt(W) + sqrt(W+1)
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by theCodeToGMAT » Wed Oct 02, 2013 9:38 am
Total time = AB/(A+B)

A = √8 + √9 = 2√2 + 3
B = √7 + √8 = √7 + 2√2

So, (2√2 + 3)(√7 + 2√2)/(2√2 + 3 + √7 + 2√2)
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by SaraLotfy » Wed Oct 02, 2013 9:58 am
Hmmm I kinda understood the question differently. Since the question says "working together" I added the weights as if it's one machine weighing W=8+7=15.

As in the rate problems where we add the rates of both machines to reach a combined rate
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by SaraLotfy » Wed Oct 02, 2013 10:00 am
theCodeToGMAT wrote:Total time = AB/(A+B)

A = √8 + √9 = 2√2 + 3
B = √7 + √8 = √7 + 2√2

So, (2√2 + 3)(√7 + 2√2)/(2√2 + 3 + √7 + 2√2)

Why do you use this equation for time -----> Total time = AB/(A+B)
when the question specifies another equation
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by theCodeToGMAT » Wed Oct 02, 2013 10:05 am
SaraLotfy wrote: Why do you use this equation for time -----> Total time = AB/(A+B)
when the question specifies another equation
This came from this formula:

1/t = 1/A + 1/B

So t = AB/(A+B)

Also, the question has given the general equation and not of total time. So we need to use the same equation for both Machines.

Sara, Do you have answer for this question?
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