A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
OA given is Op D, see in for both the Op'ns i made separate eq'ns and for Op A i got the solution but m not able to get the solution for Op B, so if someone can solve Op B that will be really helpful.
Gmat paper set Q-- probability
This topic has expert replies
Total number of bulbs = 10
Total number of defective bulbs = n
Total number of bulbs not defective = 10 - n
(1)The probability that the two bulbs to be drawn will be defective is 1/15.
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is defective = (n-1)/(10-1) = (n-1)/9
probability that the two bulbs to be drawn will be defective = (n/10) * (n-1)/9 which is equal to 1/15.
Solving the equation n(n-1) = 6
So n = 3
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is not defective = (10 - (n-1))/(10-1) = (11-n)/9
probability that one of the bulbs to be drawn will be defective and the other will not be defective = (n/10) * (11-n)/9 which is equal to 7/15.
Solving the equation n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
So answer is D
Experts, please suggest if the above approach is correct.
Total number of defective bulbs = n
Total number of bulbs not defective = 10 - n
(1)The probability that the two bulbs to be drawn will be defective is 1/15.
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is defective = (n-1)/(10-1) = (n-1)/9
probability that the two bulbs to be drawn will be defective = (n/10) * (n-1)/9 which is equal to 1/15.
Solving the equation n(n-1) = 6
So n = 3
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is not defective = (10 - (n-1))/(10-1) = (11-n)/9
probability that one of the bulbs to be drawn will be defective and the other will not be defective = (n/10) * (11-n)/9 which is equal to 7/15.
Solving the equation n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
So answer is D
Experts, please suggest if the above approach is correct.
Total number of bulbs = 10
Total number of defective bulbs = n
Total number of bulbs not defective = 10 - n
(1)The probability that the two bulbs to be drawn will be defective is 1/15.
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is defective = (n-1)/(10-1) = (n-1)/9
probability that the two bulbs to be drawn will be defective = (n/10) * (n-1)/9 which is equal to 1/15.
Solving the equation n(n-1) = 6
So n = 3
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is not defective = (10 - (n-1))/(10-1) = (11-n)/9
probability that one of the bulbs to be drawn will be defective and the other will not be defective = (n/10) * (11-n)/9 which is equal to 7/15.
Solving the equation n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
So answer is D
Experts, please suggest if the above approach is correct.
Total number of defective bulbs = n
Total number of bulbs not defective = 10 - n
(1)The probability that the two bulbs to be drawn will be defective is 1/15.
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is defective = (n-1)/(10-1) = (n-1)/9
probability that the two bulbs to be drawn will be defective = (n/10) * (n-1)/9 which is equal to 1/15.
Solving the equation n(n-1) = 6
So n = 3
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is not defective = (10 - (n-1))/(10-1) = (11-n)/9
probability that one of the bulbs to be drawn will be defective and the other will not be defective = (n/10) * (11-n)/9 which is equal to 7/15.
Solving the equation n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
So answer is D
Experts, please suggest if the above approach is correct.
-
- Master | Next Rank: 500 Posts
- Posts: 189
- Joined: Wed Jul 06, 2011 6:57 am
- Thanked: 17 times
- Followed by:1 members
GAMATO wrote:Total number of bulbs = 10
Total number of defective bulbs = n
Total number of bulbs not defective = 10 - n
(1)The probability that the two bulbs to be drawn will be defective is 1/15.
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is defective = (n-1)/(10-1) = (n-1)/9
probability that the two bulbs to be drawn will be defective = (n/10) * (n-1)/9 which is equal to 1/15.
Solving the equation n(n-1) = 6
So n = 3
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15
When first bulb is drawn, probability that it is defective = n/10
When second bulb is drawn, probability that it is not defective = (10 - (n-1))/(10-1) = (11-n)/9
probability that one of the bulbs to be drawn will be defective and the other will not be defective = (n/10) * (11-n)/9 which is equal to 7/15.
Solving the equation n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
So answer is D
Experts, please suggest if the above approach is correct.
n(11-n) = 42
Considering factors of 42, 3(11-3) = 42. So n = 3
here u are saying n = 3, how???? if u put n =3 in n(11-n), u get, 3*8 = 24
- kevincanspain
- GMAT Instructor
- Posts: 613
- Joined: Thu Mar 22, 2007 6:17 am
- Location: madrid
- Thanked: 171 times
- Followed by:64 members
- GMAT Score:790
Remember that we need to consider two equally probable cases for (2) : first is defective and second is not defective, and vice versa.
Thus 2(n/10)(10-n)/9 = 7/15
n (10 - n) = 21
n= 3 or n= 7
Since n < 5 from the question, n=3 SUFF
Thus 2(n/10)(10-n)/9 = 7/15
n (10 - n) = 21
n= 3 or n= 7
Since n < 5 from the question, n=3 SUFF
Kevin Armstrong
GMAT Instructor
Gmatclasses
Madrid
GMAT Instructor
Gmatclasses
Madrid
- 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
Statement 1 gives something away: since P(both bulbs are defective) > 0, there must be at least 2 defective bulbs.gmat25 wrote:A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
OA given is Op D, see in for both the Op'ns i made separate eq'ns and for Op A i got the solution but m not able to get the solution for Op B, so if someone can solve Op B that will be really helpful.
Since the question stems indicates that n<5, we know that the only possible values are n=2, n=3, or n=4.
Statement 1: P(both are defective) = 1/15.
If n=2, P(both are defective) = 2/10 * 1/9 = 1/45. Doesn't work.
If n=3, P(both are defective) = 3/10 * 2/9 = 1/15. This works.
Clearly, n=4 can't work, since it will increase all the numerators.
Thus, n=3.
Sufficient.
Statement 2: P(exactly 1 is defective) = 7/15.
Since only n=3 worked in statement 1, we should start with n=3.
If n=3, P(1st is defective and the 2nd is not) = 3/10 * 7/9 = 7/30.
Since P(1st is not defective and the 2nd is defective) will yield the same probability, the result above must be multiplied by 2:
P(exactly 1 is defective) = 2 * 7/30 = 7/15. This works.
Clearly n=4 can't work, since it will increase all the numerators.
Using similar logic, n=2 can't work, since it will decrease all the numerators.
Thus, n=3.
Sufficient.
The correct answer is D.
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: 189
- Joined: Wed Jul 06, 2011 6:57 am
- Thanked: 17 times
- Followed by:1 members
Thanks a lot Mitch, in Op B, i didn't multiplied with 2 and that's where i was going wrong. Thanks a lot!!!GMATGuruNY wrote:Statement 1 gives something away: since P(both bulbs are defective) > 0, there must be at least 2 defective bulbs.gmat25 wrote:A box contains 10 light bulbs, fewer than half of which are defective. Two bulbs are to be drawn simultaneously from the box. If n of the bulbs in box are defective, what is the value of n?
(1) The probability that the two bulbs to be drawn will be defective is 1/15.
(2) The probability that one of the bulbs to be drawn will be defective and the other will not be defective is 7/15.
OA given is Op D, see in for both the Op'ns i made separate eq'ns and for Op A i got the solution but m not able to get the solution for Op B, so if someone can solve Op B that will be really helpful.
Since the question stems indicates that n<5, we know that the only possible values are n=2, n=3, or n=4.
Statement 1: P(both are defective) = 1/15.
If n=2, P(both are defective) = 2/10 * 1/9 = 1/45. Doesn't work.
If n=3, P(both are defective) = 3/10 * 2/9 = 1/15. This works.
Clearly, n=4 can't work, since it will increase all the numerators.
Thus, n=3.
Sufficient.
Statement 2: P(exactly 1 is defective) = 7/15.
Since only n=3 worked in statement 1, we should start with n=3.
If n=3, P(1st is defective and the 2nd is not) = 3/10 * 7/9 = 7/30.
Since P(1st is not defective and the 2nd is defective) will yield the same probability, the result above must be multiplied by 2:
P(exactly 1 is defective) = 2 * 7/30 = 7/15. This works.
Clearly n=4 can't work, since it will increase all the numerators.
Using similar logic, n=2 can't work, since it will decrease all the numerators.
Thus, n=3.
Sufficient.
The correct answer is D.
-
- Senior | Next Rank: 100 Posts
- Posts: 33
- Joined: Wed Jan 11, 2012 5:18 pm
Hi Mitch, I don't get this. It's a selection. why are we bothered about which one is picked first? I don't know what logic I'm missing here.. I thought 3/10 * 7/9 = 7/30 is the probability for picking a defective and a good bulb.Statement 2: P(exactly 1 is defective) = 7/15.
Since only n=3 worked in statement 1, we should start with n=3.
If n=3, P(1st is defective and the 2nd is not) = 3/10 * 7/9 = 7/30.
Since P(1st is not defective and the 2nd is defective) will yield the same probability, the result above must be multiplied by 2:
P(exactly 1 is defective) = 2 * 7/30 = 7/15. This works.
Clearly n=4 can't work, since it will increase all the numerators.
Using similar logic, n=2 can't work, since it will decrease all the numerators.
Thus, n=3.
Sufficient.
thanks.
- 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
We have to account for ALL of the different ways to get a favorable outcome.nailGmat2012 wrote:Hi Mitch, I don't get this. It's a selection. why are we bothered about which one is picked first? I don't know what logic I'm missing here.. I thought 3/10 * 7/9 = 7/30 is the probability for picking a defective and a good bulb.Statement 2: P(exactly 1 is defective) = 7/15.
Since only n=3 worked in statement 1, we should start with n=3.
If n=3, P(1st is defective and the 2nd is not) = 3/10 * 7/9 = 7/30.
Since P(1st is not defective and the 2nd is defective) will yield the same probability, the result above must be multiplied by 2:
P(exactly 1 is defective) = 2 * 7/30 = 7/15. This works.
Clearly n=4 can't work, since it will increase all the numerators.
Using similar logic, n=2 can't work, since it will decrease all the numerators.
Thus, n=3.
Sufficient.
thanks.
If 3 of the bulbs are defective and 7 are not, there are TWO WAYS to select EXACTLY ONE defective bulb:
Case 1: the first bulb is defective, the second bulb is not
P(the first bulb is defective) * P(the second bulb is not defective) = 3/10 * 7/9 = 7/30.
Case 2: the first bulb is not defective, the second bulb is defective
P(the first bulb is not defective) * P(the second bulb is defective) = 7/10 * 3/9 = 7/30.
Since either Case 1 OR Case 2 will yield a favorable outcome, we ADD the probabilities:
7/30 + 7/30 = 14/30 = 7/15.
The key word here is EXACTLY: whenever we have to determine the probability that something happens EXACTLY n times, we must account for ALL of the different ways to get a favorable outcome.
To find similar problems, type "probability", "exactly" and "gmatguruny" into the search bar.
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