Lights on.. lights off

Expert replies

dumb.doofus: Master | Next Rank: 500 Posts; Posts: 435; Joined: Sat Sep 27, 2008 2:02 pmFollowed by: 1 member; Location: San Jose, CA; Thanked: 43 times; GMAT Score:720

by dumb.doofus » Mon May 18, 2009 10:31 pm

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?

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

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Source: Beat The GMAT — Problem Solving | Mon May 18, 2009 10:31 pm

scoobydooby: Legendary Member; Posts: 1035; Joined: Wed Aug 27, 2008 10:56 pmFollowed by: 1 member; Thanked: 104 times

by scoobydooby » Mon May 18, 2009 10:42 pm

am getting 10%

set it up in a 2 by 2 matrix.
let total lights be 100, 80 actually switched on, 20 actually switched off.
let x be expected to be switched off, 100-x expected to be switched on.

0.4x+0.9(100-x)=80.....1
solving we get x=20

expected to be off/actually on=0.4*20/80=10%

hence, D

Svedankae: Master | Next Rank: 500 Posts; Posts: 101; Joined: Tue Apr 07, 2009 12:30 am; Thanked: 2 times; GMAT Score:750

by Svedankae » Tue May 19, 2009 1:53 am

what would that 2x2 matrix for yours look like?

scoobydooby: Legendary Member; Posts: 1035; Joined: Wed Aug 27, 2008 10:56 pmFollowed by: 1 member; Thanked: 104 times

by scoobydooby » Tue May 19, 2009 2:02 am

here you go

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dumb.doofus: Master | Next Rank: 500 Posts; Posts: 435; Joined: Sat Sep 27, 2008 2:02 pmFollowed by: 1 member; Location: San Jose, CA; Thanked: 43 times; GMAT Score:720

by dumb.doofus » Tue May 19, 2009 3:38 pm

Well done, ScoobyDooby..

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