Eighty percent of the lights at Hotel California are on at 8 : PS Archive
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# Eighty percent of the lights at Hotel California are on at 8

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Manager
Joined: 07 May 2007
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Eighty percent of the lights at Hotel California are on at 8 [#permalink]

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16 Jul 2007, 17:06
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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?
a) 22(2/9)%
b) 16(2/3)%
c) 11(1/9)%
d) 10%
e) 5%
Director
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Re: PS - Percentage of lights [#permalink]

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16 Jul 2007, 17:57
iamba wrote:
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?
a) 22(2/9)%
b) 16(2/3)%
c) 11(1/9)%
d) 10%
e) 5%

I'm doing something wrong on this one. who can help?
80/100 lights are on at HC. .4(80) = 32 lights should be off.
20/100 lights are off at HC. .1(20) 2 lights should be on
32 should be off/80 are on = 40%
Intern
Joined: 06 Apr 2007
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16 Jul 2007, 18:53
Let x be the percentage of lights supposed to be off

40%*x + 90% (1-x) = 80%
5000x = 1000
x = 1/5 = 20% ............... 20% of ALL lights are supposed to be off

40 percent of the lights that are supposed to be off are actually on

40% x 20% = 8%

8% / 80% = 10%

(D) 10%
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Joined: 05 Mar 2007
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17 Jul 2007, 02:14
cocosdreamland wrote:
Let x be the percentage of lights supposed to be off

40%*x + 90% (1-x) = 80%
5000x = 1000
x = 1/5 = 20% ............... 20% of ALL lights are supposed to be off

40 percent of the lights that are supposed to be off are actually on

40% x 20% = 8%

8% / 80% = 10%

(D) 10%

Thanks!

I got lost for a while!

FP
17 Jul 2007, 02:14
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