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# Eighty percent of the lights at Hotel California are on at 8 p.m.

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Eighty percent of the lights at Hotel California are on at 8 p.m. [#permalink]

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02 Nov 2008, 08:56
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Question Stats:

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Eighty percent of the lights at Hotel California are on at 8 p.m. a certain evening. However, forty percent of the lights that are supposed to be off are actually on and ten percent of the lights that are supposed to be on are actually off. What percent of the lights that are on are supposed to be off?

22(2/9)%
16(2/3)%
11(1/9)%
10%
5%

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Current Student
Joined: 28 Dec 2004
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Schools: Wharton'11 HBS'12
Re: Eighty percent of the lights at Hotel California are on at 8 p.m. [#permalink]

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02 Nov 2008, 09:37
LiveStronger wrote:
Source:MGMAT CAT

Eighty percent of the lights at Hotel California are on at 8 p.m. a certain evening. However, forty percent of the lights that are supposed to be off are actually on and ten percent of the lights that are supposed to be on are actually off. What percent of the lights that are on are supposed to be off?

22(2/9)%
16(2/3)%
11(1/9)%
10%
5%

i took nyquil last night and my brain is forzen but mmaybe I am wrong and cant figure out

but say we have 100 lights=80 are on+20 are off

40% of 20 are on i.e 8 are on..which means 8/80*100 or 10% of the ligths that should be off are ON..

d it is

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Intern
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Re: Eighty percent of the lights at Hotel California are on at 8 p.m. [#permalink]

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02 Nov 2008, 09:44
I'd pick Answer as D (10%)

Typical percent problem, so take total # of lights as 100. Now 80% of lights are currently ON or 80 lights

# of lights supposed to be ON = N
# of lights supposed to be OFF = F

Rewriting the sentence, you'd get (40/100)*F + N - (10/100)*N = 80.
Also, by our 100 light assumption, we can state F + N = 100

solving for F and N, you get F = 20 and N = 80. Hmmm...now I almost made a mistake here.
If you substitute this back, # of lights supposed to OFF that are ON = (40/100)* 20 which is 8. And # of lights supposed to ON that are OFF = (10/100)* 80 which is also 8.

anyways, the question is what % of lights that are ON that should be OFF.. so (8/80) * 100, which leaves us with a final answer that Hotel California is Energy In-effecient by 10%
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Senior Manager
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Location: Motortown
Re: Eighty percent of the lights at Hotel California are on at 8 p.m. [#permalink]

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03 Nov 2008, 08:07
OA is D

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Re: Eighty percent of the lights at Hotel California are on at 8 p.m.   [#permalink] 03 Nov 2008, 08:07
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