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MGMAT question help with the strategy? pls..

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laklucky
 Post subject: MGMAT question help with the strategy? pls..
 Post Posted: Thu Sep 02, 2010 2:03 pm 
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Students


Posts: 11
Eighty percent of the lights at Hotel California are switched on at 8 p.m. one evening. However, forty percent of the lights that are supposed to be switched off are actually switched on, and ten percent of the lights that are supposed to be switched on are actually switched off. What percent of the lights that are switched on are supposed to be switched off?

I solved it like this: Please let me know where I am going wrong or whether the whole methodology itself is wrong

L = Loff+Lon

(80/100)L = (40/100)Loff - (10/100)Lon + Lon

8L = 4Loff - L + Loff +10L -10Loff

17L = -5Loff

Required [(40/100)Loff]/[(80/100)L] * 100

I am getting the solution as hideous 170% in this way where as the answer is 10%. Where am I going wrong?
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laklucky
 Post subject: Re: MGMAT question help with the strategy? pls..
 Post Posted: Thu Sep 02, 2010 2:11 pm 
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Students


Posts: 11
laklucky wrote:
Eighty percent of the lights at Hotel California are switched on at 8 p.m. one evening. However, forty percent of the lights that are supposed to be switched off are actually switched on, and ten percent of the lights that are supposed to be switched on are actually switched off. What percent of the lights that are switched on are supposed to be switched off?

I solved it like this: Please let me know where I am going wrong or whether the whole methodology itself is wrong

L = Loff+Lon

(80/100)L = (40/100)Loff - (10/100)Lon + Lon

8L = 4Loff - L + Loff +10L -10Loff

17L = -5Loff

Required [(40/100)Loff]/[(80/100)L] * 100

I am getting the solution as hideous 170% in this way where as the answer is 10%. Where am I going wrong?



Sorry stupid calculation mistake :

-L = -5Loff is what I get and that gives the correct solution!
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mschwrtz
 Post subject: Re: MGMAT question help with the strategy? pls..
 Post Posted: Fri Sep 03, 2010 3:29 am 
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ManhattanGMAT Staff


Posts: 506
Good catch, then.
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