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Eighty percent of the lights at Hotel California are

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Eighty percent of the lights at Hotel California are [#permalink] New post 06 May 2006, 21:30
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Eighty percent of the lights at Hotel California are supposed to be 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?
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 [#permalink] New post 06 May 2006, 22:20
8% ACCORDING TO ME.

20% OF THE LIGHTS ARE SUPPOSSED TO BE "OFF", OUT OF WHICH 40% IS ACTUALLY "ON" I.E, 8 (20*0.4) LIGHTS ARE ON. HENCE 8% OF THE LIGHTS ARE ON WHICH ARE SUPPOSSED TO BE OFF.
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 [#permalink] New post 07 May 2006, 06:24
10% for me
lets check
100 lamps in total
80 supposed to be on and 20 off
but 40% of 20 are on-->8 lamps
and 10% of 80 are off-->8 lamps
so finally 80 lamp remain on
What percent of the lights that are on are supposed to be off
8 lamps off 80 supposed to be off answer is 10%
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Senior Manager
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 [#permalink] New post 07 May 2006, 07:04
yep 10% is correct answer! though I was extremely confuse with

"What percent of the lights that are on are supposed to be off "

I came up with 0 as answer!

because previously there were 80% on light and now also 80%!
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 [#permalink] New post 07 May 2006, 07:26
chiragr wrote:
yep 10% is correct answer! though I was extremely confuse with

"What percent of the lights that are on are supposed to be off "

I came up with 0 as answer!

because previously there were 80% on light and now also 80%!


80% of the lights are expected to ON. If you do the calculations you would find that 80% of the lights ARE STILL ON. So 0% ought to be the answer!!!

What is the source of the question? Nice one.
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 [#permalink] New post 07 May 2006, 12:27
Lets assume there are 100 lights. 80 (80n) supposed to be on and 20 (20f) supposed to be off.

..where n just means on and f means off

10% of 80n are off .. so 8n lights are off

40% of 20f are on .. so 8f are on

So, total lights that are on = 72n + 8f = 80 lights.

What percent of the lights that are on are supposed to be off?

8/80 * 100 = 10% .. yea that should be it.
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 [#permalink] New post 08 May 2006, 08:34
It is indeed 10%
Assume:
there are 100 lights.
80 - lights are supposed to be on
20 - lights are supposed to be off.
8 - (20*0.4) - 40% of the lights that are supposed to be off are on
8 - (80*0.1) – 10% of the lights that are supposed to be on are off
the question asks what % of the lights that are on are supposed to be off
the lights that are on – 80
the lights that are supposed to be off, but on - 8
hence, the answer is 80 - 100%, 8 – 10%
  [#permalink] 08 May 2006, 08:34
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