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Math problem:trun on turn off lights.

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tianyuwu.christina
 Post subject: Math problem:trun on turn off lights.
 Post Posted: Thu Feb 25, 2010 4:30 pm 
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Students


Posts: 1

This type of problems are very comfusing, and I'm wondering are there any way to simplify the question? (FROM THE CAT EXAM)

Eighty percent of the lights at Hotel California are switched on at 8:00pm one evening. However, forty percent of the lights that are supposed to be switched off are actually swtiched on, and ten percent of the lights that are supposed to swtiched on are actually swtiched off. What percent of the lights that are switched on are suppose to be switched off?

A. 22(2/9)%
B. 16(2/3)%
C. 11(1/9)%
D. 10%
E. 5%
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Ben Ku
 Post subject: Re: Math problem:trun on turn off lights.
 Post Posted: Tue Mar 16, 2010 3:01 am 
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ManhattanGMAT Staff


Posts: 824
You stated that this is from the CAT exam. Which CAT exam is it from? Was it one of Manhattan GMAT's CAT Exams, or another company? We can't answer the question unless we have the source (author) of the problem.

_________________
Ben Ku
Instructor
ManhattanGMAT
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jaygmat
 Post subject: Re: Math problem:trun on turn off lights.
 Post Posted: Tue Mar 16, 2010 9:35 am 
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Students


Posts: 1
we need to find out the percent of lights that used to be previously switched on

let n= percent of lights switched on
f=percent of lights switched off

n+f = 100

n+.2f-.1n=80

solve for n, will give you n = 88.88

percent of lights used to be switched on but are switch of = 8.88/80*100 = 11.111
= 11 1/9
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sen.sreeroop
 Post subject: Re: Math problem:trun on turn off lights.
 Post Posted: Thu Mar 25, 2010 3:00 am 
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Forum Guests


Posts: 4
Jay,

can you please help me understand how did you get .2f and .1n in the second equation?

Thanks in advance...
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