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 is A
How many Bulbs are ON?
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 183
- Joined: Sat Apr 16, 2011 11:29 am
- Thanked: 7 times
- Followed by:2 members
- manpsingh87
- Master | Next Rank: 500 Posts
- Posts: 436
- Joined: Tue Feb 08, 2011 3:07 am
- Thanked: 72 times
- Followed by:6 members
let lights that were suppose to be on be represented by a and lights that were suppose to be off be represented by b, also assume total no. of lights to be x;Chaitanya_1986 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 is A
now as per the given condition; 80% of the lights are ON; which comprises of 90% of a and 40% of b;
.9a+.4b=.8x;....1)
as 80% of the lights are ON; therefore remaining 20% are off; which comprises of 10% of a and 60% of b;
hence .1a+.6b=.2x;---2)
multiply 2 with 9 we have;
.9a+5.4b=1.8x;
now subtract 1 from it we have;
5b=x;
put x=5b in 1 we have;
.9a=3.6b;
a=4b;
therefore; required percentage is
(.4b/4b)*100=10; hence A
O Excellence... my search for you is on... you can be far.. but not beyond my reach!
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
Like many GMAT problems, the one above can be solved with clever reasoning and perhaps some trial and error.Chaitanya_1986 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 is A
Let total bulbs = 100.
Number of bulbs on = .8*100 = 80.
Since 80 bulbs are on, and 90% of the bulbs expected to be on are in fact on, the number of bulbs expected to be on must be very high.
90 bulbs expected to be on would be too high, because with 10% of these switched off, the number left on would be 90-9 = 81, which is too many.
Let bulbs expected to be on = 80.
10% of these off = 8 off, 72 on.
Bulbs expected to be off = 100-80 = 20.
40% of these on = 8 on, 12 off.
Total on = 72+8 = 80. This works.
Thus, of the 80 bulbs that are on, 8 were expected to be off.
8/80 = 10%.
The correct answer is A.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
-
- Master | Next Rank: 500 Posts
- Posts: 135
- Joined: Tue Oct 13, 2009 10:27 am
- Thanked: 3 times
Hi manpsingh87,
You wrote: now as per the given condition; 80% of the lights are ON; which comprises of 90% of a and 40% of b. Can you tell me where are we getting the 90% and the 40% from?
Thanks,
B
You wrote: now as per the given condition; 80% of the lights are ON; which comprises of 90% of a and 40% of b. Can you tell me where are we getting the 90% and the 40% from?
Thanks,
B
- manpsingh87
- Master | Next Rank: 500 Posts
- Posts: 436
- Joined: Tue Feb 08, 2011 3:07 am
- Thanked: 72 times
- Followed by:6 members
read the stem of the question carefully.. it states..!!!boazkhan wrote:Hi manpsingh87,
You wrote: now as per the given condition; 80% of the lights are ON; which comprises of 90% of a and 40% of b. Can you tell me where are we getting the 90% and the 40% from?
Thanks,
B
which means, lights that are on are 40% of b; and 90% of a (as 10% are off)...!!!If 40% of lights that were expected to be off, were in fact on, and 10% of lights that were expected to be on
O Excellence... my search for you is on... you can be far.. but not beyond my reach!
- GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Joined: Tue May 25, 2010 12:04 pm
- Location: New York, NY
- Thanked: 13060 times
- Followed by:1906 members
- GMAT Score:790
To solve algebraically, I would use the following approach:boazkhan wrote:Hi manpsingh87,
You wrote: now as per the given condition; 80% of the lights are ON; which comprises of 90% of a and 40% of b. Can you tell me where are we getting the 90% and the 40% from?
Thanks,
B
Assume 100 bulbs.
Let x = bulbs expected to be off.
Since 40% of these bulbs are on, the number on = .4x.
Let 100-x = bulbs expected to be on.
Since 10% of these bulbs are off, the number on = .9(100-x).
Since 80 bulbs are on, we get:
.4x + .9(100-x) = 80
4x + 900 - 9x = 800
-5x = -100
x = 20.
Thus, 20 bulbs were expected to be off.
Of these 20 bulbs, the number on = .4*20 = 8.
Thus, of the 80 bulbs that are on, the percentage that were expected to be off = 8/80 = 10%.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
- sl750
- Master | Next Rank: 500 Posts
- Posts: 496
- Joined: Tue Jun 07, 2011 5:34 am
- Thanked: 38 times
- Followed by:1 members
Yes, it can be solved by the table method
Expected On Expected Off
Light On .9(y) .4(x) 80
Light Off .1(y) .6(x) 20
------------ ------------- ---
y x 100
.9y+.4x = 80
.1y+.6x = 20
Solve for x. You get x=20. Therefore (.4(20)/80)*100 = 10
Expected On Expected Off
Light On .9(y) .4(x) 80
Light Off .1(y) .6(x) 20
------------ ------------- ---
y x 100
.9y+.4x = 80
.1y+.6x = 20
Solve for x. You get x=20. Therefore (.4(20)/80)*100 = 10