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# If machine A and machine B can finish the task in 4 hours when working

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Intern
Joined: 18 Apr 2013
Posts: 34
If machine A and machine B can finish the task in 4 hours when working  [#permalink]

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06 Jul 2017, 06:59
2
00:00

Difficulty:

15% (low)

Question Stats:

74% (00:50) correct 26% (01:13) wrong based on 93 sessions

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If machine A and machine B can finish the task in 4 hours when working together at their constant rates, in how many hours can machine B finish the task alone?
1) Machine A can finish the task in 6 hours alone
2) The hours that it takes machine B to finish the task alone is 6 hours longer than the hours that it takes machine A to finish the task alone
Manager
Joined: 23 Jul 2015
Posts: 156
Re: If machine A and machine B can finish the task in 4 hours when working  [#permalink]

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06 Jul 2017, 09:53
Given: Ra +Rb = 1/4

A. Ra= 1/6 --> 1/6 + 1/Tb = 1/4 --> Suff
B. Tb = Ta + 6
Rb = 1/Tb
Ra = 1/(Tb -6)
1/(Tb-6) + 1/Tb = 4
==> (Tb-12)(Tb-2)=0
Tb=2 is invalid for Ta cannot be negative (Ta=Tb -6)
Ans D
Intern
Joined: 04 Jan 2016
Posts: 36
Location: United States
Concentration: General Management
Schools: Tuck '20 (M)
GMAT 1: 750 Q48 V44
GPA: 3.3
Re: If machine A and machine B can finish the task in 4 hours when working  [#permalink]

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06 Jul 2017, 11:08
By the work equation, we have

$$\frac{1}{x} + \frac{1}{y} = \frac{1}{4}$$

Since the question is asking for $$y$$, we only need to eliminate one of the variables to solve the problem.

(1) $$\frac{1}{6} + \frac{1}{y} = \frac{1}{4}$$

Sufficient: This gives us $$x$$, so we can now solve for $$y$$.

(2) $$\frac{1}{x} + \frac{1}{x+6} = \frac{1}{4}$$

Sufficient: This gives us $$y$$ in terms of $$x$$ so we can now solve for $$x$$ and substitute to solve $$y = x + 6$$.

Therefore, D
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Joined: 09 Sep 2013
Posts: 8803
Re: If machine A and machine B can finish the task in 4 hours when working  [#permalink]

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26 Oct 2018, 03:56
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Re: If machine A and machine B can finish the task in 4 hours when working &nbs [#permalink] 26 Oct 2018, 03:56
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