Math problem:trun on turn off lights.

[tianyuwu.christina]

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%

[Ben Ku]

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.

[jaygmat]

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

[sen.sreeroop]

Jay,

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

Thanks in advance...