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work problem

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by smallsorrow » Mon Sep 22, 2008 7:15 am
Working alone, printers x, y and z can do a certain printing job in 12, 15 an 18 hours, respectively. What is the ratio of the time it takes the printer x to do the job, working alone, to the time it takes y and z to do the job, working together at their individual rates?
a) 4/11
B) 1/2
C) 15/22
D) 22/15
E) 11/4


OA follows.

I don´t understand why to flip the fraction in the end...
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by tendays2go » Mon Sep 22, 2008 7:20 am
y and z take 15 and 18 hrs respectively,
so working together they should complete the work between
7.5 and 9 hrs
by the formula, it is: (15*18 )/ (15+18 ) = 90/11

now, answer is: 12/ (90/11 ) = 22/15
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Re: work problem

by stop@800 » Mon Sep 22, 2008 7:37 am
smallsorrow wrote:Working alone, printers x, y and z can do a certain printing job in 12, 15 an 18 hours, respectively. What is the ratio of the time it takes the printer x to do the job, working alone, to the time it takes y and z to do the job, working together at their individual rates?
a) 4/11
B) 1/2
C) 15/22
D) 22/15
E) 11/4


OA follows.
I think the answer is D

I don´t understand why to flip the fraction in the end...
I am not sure what flipping are you talking about.

One flipping is in the ratio and your denom is a fraction.

Another is while calculation rate of y + z
in this you get inverse of cobmined rate by adding inverse of individual rates

an easy one will be
combined rate of yz = (y * z) / (y + z)
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