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Lois can complete a certain job in x hours and Parnel can complete the

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Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 04 Apr 2017, 05:28
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A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

98% (00:27) correct 2% (01:59) wrong based on 57 sessions

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Lois can complete a certain job in x hours and Parnel can complete the same job in y hours. Which of the following expresses, in terms of x and y, the time it would take them to complete the job if they worked together?

A. (x + y)/y
B. xy/y
C. xy/x
D. 2/(xy)
E. xy/(x + y)

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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 04 Apr 2017, 05:47
Answer is E

1/x + 1/y=Combined rate
time taken would be xy/(x+y)
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Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 04 Apr 2017, 06:42
Bunuel wrote:
Lois can complete a certain job in x hours and Parnel can complete the same job in y hours. Which of the following expresses, in terms of x and y, the time it would take them to complete the job if they worked together?

A. (x + y)/y
B. xy/y
C. xy/x
D. 2/(xy)
E. xy/(x + y)



E.
Pick numbers

Lois
Job in 10 hours
X=10
\(\frac{1}{10}\) job an hour

Parnel
Job in 5 hours
Y =5
\(\frac{1}{5}\) job an hour

Together
x + y = \(\frac{3}{10}\) job an hour
or 3.33 hours to do the job together.

Plug in x=10 and y=5 into the answer choices until you find 3.33 which is E.
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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 04 Apr 2017, 07:02
1/x + 1/y (t) = 1

(x+y)/xy * (t) = 1

t = xy/(x+y)

E


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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 04 Apr 2017, 12:55
1/x + 1/y
y +x/xy
Flip over the fraction

xy/y + x

Answer is E
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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 18 Aug 2018, 06:36
WORK = TIME x RATE
WORK = 1
T(L)=X, R(L)=1/X
T(P)=Y, R(P)=1/Y

Rate Together= 1/x+1/y= (y+x)/xy
Time together= Work/Rate=1/((y+x)/xy)= xy/(y+x)
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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 18 Aug 2018, 07:29
Bunuel wrote:
Lois can complete a certain job in x hours and Parnel can complete the same job in y hours. Which of the following expresses, in terms of x and y, the time it would take them to complete the job if they worked together?

A. (x + y)/y
B. xy/y
C. xy/x
D. 2/(xy)
E. xy/(x + y)

Let th total work be xy units

The efficiency of Lois is \(y\) units/hr
The efficiency of Parnel is \(x\) units/hr

Combined efficiency of Lois & Parnel is \((x + y)\) units/hr

So, the time required by Lois & Parnel is \(\frac{xy}{(x + y)}\)

Answer must be (E)


PS : THIS IS BASICALLY A FORMULA, KNOWING THIS FORMULA CAN HELP DURING THE EXAM
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Re: Lois can complete a certain job in x hours and Parnel can complete the  [#permalink]

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New post 18 Aug 2018, 09:08
rate of Lois = 1/x
rate of Parnel = 1/y
combined rate = 1/x + 1/y = x+y/xy

so time = 1/rate = xy/x+y

Answer is E
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Re: Lois can complete a certain job in x hours and Parnel can complete the &nbs [#permalink] 18 Aug 2018, 09:08
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