eshwarjayanth wrote:80% of the lights in a hotel were on at 8.00 pm on some evening. If 40% of lights that were expected to be off, were in fact on, and 10% of lights that were expected to be on, were in fact off; what percent of the lights that are on, are the lights that were not expected to be on?
A. 10
B. 12
C. 100/9
D. 8
E. 18
OA - A
100 bulbs
80 are ON
20 are OFF
if x lights were expected to be OFF and out of X, 0.4x are ON
So total OFF =
0.6x
So 0.4X are the lights that are ON and were not expected to be ON. --
WE HAVE TO FIND THIS
also,
if Y lights were expected to be ON and out of Y, 0.1Y are OFF
so total ON =
0.9Y
Total ON = 0.4X+0.9Y =80, 4X+9Y =800
and total OFF = 0.6x+0.1Y = 20, 6x+y= 200
or,
54x+9y =1800 (multiply 9 both sides)
4x+9y = 800
Subtracting the equations we will get
50x = 1000
x= 20
0.4X = 8
8/80 = 10%
