Defective roll probability

Expert replies

jcnissi: Junior | Next Rank: 30 Posts; Posts: 18; Joined: Sat Jun 12, 2010 10:17 pm; Thanked: 1 times

by jcnissi » Thu Nov 18, 2010 8:34 pm

There are 15 rolls of film in a box and 3 are defective. Two rolls are to be selected, one after the other. What is the probability of selecting a defective roll followed by another defective roll?

Like this post

Source: Beat The GMAT — Problem Solving | Thu Nov 18, 2010 8:34 pm

GMAT/MBA Expert

Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Thu Nov 18, 2010 9:25 pm

jcnissi wrote:There are 15 rolls of film in a box and 3 are defective. Two rolls are to be selected, one after the other. What is the probability of selecting a defective roll followed by another defective roll?

Defective number of rolls = 3
Total number of rolls = 15
Probability of selecting a defective roll out of 15 = 3/15

Now, Defective number of rolls = 2
Total number of rolls = 14
Probability of selecting a defective roll out of 14 = 2/14

Required probability = (3/15)*(2/14) = 1/35

Last edited by Rahul@gurome on Fri Nov 19, 2010 5:03 am, edited 1 time in total.

Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)

Like this post

hoji: Master | Next Rank: 500 Posts; Posts: 114; Joined: Fri Oct 08, 2010 10:35 amFollowed by: 2 members; Location: TASHKENT; Thanked: 8 times; GMAT Score:640

by hoji » Fri Nov 19, 2010 4:45 am

Rahul, i think 3 are defective of those 15 rolls, so my solution is:
probability of coming the first roll out as a defective is 3/15 and consequently the second one`s is 2/14 and taking them together, it is like 3/15*2/14=1/35
what do you say?

Like this post

GMAT/MBA Expert

Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Fri Nov 19, 2010 5:04 am

hoji wrote:Rahul, i think 3 are defective of those 15 rolls, so my solution is:
probability of coming the first roll out as a defective is 3/15 and consequently the second one`s is 2/14 and taking them together, it is like 3/15*2/14=1/35
what do you say?

Thanks hoji!
Edited the reply.

Rahul Lakhani
Quant Expert
Gurome, Inc.
https://www.GuroMe.com
On MBA sabbatical (at ISB) for 2011-12 - will stay active as time permits
1-800-566-4043 (USA)
+91-99201 32411 (India)

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review