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A chemical lab is scheduled to go through two air ventillati

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A chemical lab is scheduled to go through two air ventillati  [#permalink]

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New post Updated on: 11 Sep 2013, 02:13
2
00:00
A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

89% (01:37) correct 11% (02:44) wrong based on 119 sessions

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A chemical lab is scheduled to go through two air ventillation systems, a basic system and a more advanced system that takes half an hour and two hours respectively to fully ventillate the room. If both ventillation systems were to operate at the same time with thier respective constant rates, how many hours would it take to fully ventilate the chemical lab?

(A) 1/4
(B) 1/3
(C) 2/5
(D) 5/4
(E) 3/2

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Originally posted by amgelcer on 10 Sep 2013, 17:55.
Last edited by Bunuel on 11 Sep 2013, 02:13, edited 1 time in total.
Edited the question.
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Re: A chemical lab is scheduled to go through two air ventillati  [#permalink]

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New post 10 Sep 2013, 18:30
2
amgelcer wrote:
A chemical lab is scheduled to go through two air ventillation systems, a basic system and a more advanced system that takes half an hour and two hours respectively to fully ventillate the room. If both ventillation systems were to operate at the same time with their respective constant rates, how many hours would it take to fully ventilate the chemical lab?

(A) 1/4
(B) 1/3
(C) 2/5
(D) 5/4
(E) 3/2

I'm happy to respond. :-)

As I discuss in this post,
http://magoosh.com/gmat/2012/gmat-work-rate-problems/
always add rates when machines work together.

basic system = A --- ventilates in 0.5 hr, so A's rate is (1 room)/(0.5 hour) = 2

more advanced system = B --- ventilates in 2 hr, so A's rate is (1 room)/(2 hour) = 0.5

Add rate --- combined rate = 2 + 0.5 = 2.5 rooms/hour = 5/2 rooms/hour

Now, how long would it take for a combo, ventilating at 5/2 rooms/hour, to ventilate a single room? We use the "ART" equation ----

A = RT --- that is, (amount) = (rate)*(time)

(1 room) = (5/2 rooms/hour)*T

1 = (5/2)*T

2/5 = T

So, the combo takes 2/5 of an hour to ventilate a room. Answer = (C)

Does this make sense?
Mike :-)
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Re: A chemical lab is scheduled to go through two air ventillati  [#permalink]

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New post 11 Sep 2013, 00:13
If you had to guess on this question, there are some ways to logically knock out a bunch of answers.

The two ventilation systems can clear the room in 1/2 hour and 2 hours, so combined they have to take less than 1/2 hour to do the job. Answers D and E are too big. Also, if both machines could do the job in 1/2 hour, then it would take 1/4 hour combined. Since the second machine takes 2 hours (and not 1/2 hour), then answer A is too small.

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Re: A chemical lab is scheduled to go through two air ventillati  [#permalink]

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New post 11 Sep 2013, 03:21
1
Basic System, B takes 1/2 hour to do 1 work
Advanced System, A takes 2 hours to do 1 work

In 1 hour, B will do 2 times work
In 1 hour, A will do 1/2 of work

Working together, in 1 hour, A & B will do 2 + 1/2 = 5/2 times of Work
So, they will finish the work in 2/5 Hours.
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Re: A chemical lab is scheduled to go through two air ventillati  [#permalink]

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New post 17 Sep 2018, 21:29
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

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Re: A chemical lab is scheduled to go through two air ventillati &nbs [#permalink] 17 Sep 2018, 21:29
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