probability

GMATQuantCoach: Senior | Next Rank: 100 Posts; Posts: 44; Joined: Sun Jun 28, 2009 8:58 am; Location: Online; Thanked: 13 times; GMAT Score:51

by GMATQuantCoach » Sun Jun 28, 2009 10:47 pm

For the sum to be odd, the three balls must be odd,odd,odd, or even,even,odd, or even, odd, even, or odd, even, even

The probability of each case is 1/8 ( 1/2 * 1/2 * 1/2).

Then total probability is 4 * 1/8 = 1/2

FREE Weekly Online Office Hours

Upcoming FREE Online Class: Effective Calculations - 11/15/09 Sunday

Instructor | GMATPrepMath.com | GMAT Prep Courses Starting at $60 with FREE GMAT Math Formula Sheet

Choose from Topics:
Number Properties - Advanced Counting (Combinatorics) - Probability - Advanced Algebra in DS - Geometry - Word Problems - Fundamental Algebra in PS - Effective Calculations

Quote

GMAT/MBA Expert

Ian Stewart: GMAT Instructor; Posts: 2621; Joined: Mon Jun 02, 2008 3:17 am; Location: Montreal; Thanked: 1090 times; Followed by:355 members; GMAT Score:780

by Ian Stewart » Tue Jun 30, 2009 9:54 am

Alternatively: it doesn't matter what your first two selections are. They either add to an even number, in which case you need the third ball to be odd (probability 1/2) or they add to an odd number, in which case you need the third ball to be even (probability 1/2). No matter what the situation after your first two selections, there's a 1/2 probability you'll arrive at an odd sum after your third selection.

For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com

Quote

gmat_2010: Senior | Next Rank: 100 Posts; Posts: 50; Joined: Thu Apr 09, 2009 10:33 am

by gmat_2010 » Thu Jul 30, 2009 7:16 am

Yup, I arrived at the solution the same way Ian did. Probability of any ball being odd is same of that being even. And the total sum can be only either even or odd. Hence, there is no preference for even or odd. Hence both are equally likely. Exhaustive set. Hence, 1/2 for each.

Quote

Post new topic Post Reply

• Page 1 of 1

5/5

5 Star (473 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