BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Probability (Marbles in a bag)

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Mon Mar 19, 2012 3:47 am

Probability (Marbles in a bag)

by ashfu2002 » Tue Apr 03, 2012 9:22 am
A bag contains some red and some blue marbles.How many red marbles are in the bag?

(1) If two marbles are selected from the bag, the probability of selecting one red and one blue marble is 1/4

(2) There are a total of 8 marbles in the bag.

OA E
Top
Source: — Data Sufficiency |
User avatar
GMAT Instructor
Posts: 1248
Joined: Thu Mar 29, 2012 2:57 pm
Location: Everywhere
Thanked: 503 times
Followed by:192 members
GMAT Score:780

by Bill@VeritasPrep » Tue Apr 03, 2012 9:48 am
1) There are two ways of getting one blue and one red, each of which has the same probability:

B-R
R-B

Since the total probability is 1/4, P(B-R)=1/8 and P(R-B)=1/8. If we have 7 red and 1 blue, then P(R-B) = 7/8 * 1/7 = 7/56 = 1/8. P(B-R) would have the same probability for a total of 1/4. However, we can also arrive at the same probability if we have 7 blue and 1 red. Insufficient.

2) R + B = 8. There are many possible combinations of red and blue marbles. 1 and 7, 2 and 6, and so on. Insufficient.

When put together, we still have the same possibilities we had in Statement 1, so we still cannot answer.
Join Veritas Prep's 2010 Instructor of the Year, Matt Douglas for GMATT Mondays

Visit the Veritas Prep Blog

Try the FREE Veritas Prep Practice Test
Top
Post new topic Post Reply
• Page 1 of 1