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

Redeem

MGMAT probability question (BAFFLED!!!)

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 347
Joined: Mon Aug 04, 2008 1:42 pm
Thanked: 1 times

MGMAT probability question (BAFFLED!!!)

by Stockmoose16 » Mon Sep 08, 2008 2:28 pm
In a bag of marbles, there are 3 red, 2 white, and 5 blue. If Bob takes 2 marbles out of the bag, what is the probability that he will have one white and one blue marble?


This is from the MGMAT book, and the key says the answer is 2/9. Shouldn't it just be:

2/10 (chances of getting a white on first pick) * 5/9 (chances of getting blue on second pick):

10/100 = 1/9

Why is this different than a typical marble out of a hat questions (with no replacement?)?
Last edited by Stockmoose16 on Tue Sep 09, 2008 2:25 pm, edited 1 time in total.
Top
Source: — Problem Solving |
Senior | Next Rank: 100 Posts
Posts: 50
Joined: Tue Mar 11, 2008 6:30 am
Thanked: 6 times

by bourne159 » Mon Sep 08, 2008 2:35 pm
You got one half of the answer. You forgot the other part.
The question did not say find the probability of first marble being white and the second marble being Blue.

It jut asked for probability of picking up a white marble and a blue marble.

Probability of picking one white and one blue = P(W). P(B) + P (B). P(W)
2/10.5/9 + 5/10.2/9 = 1/9 + 1/9 = 2/9
Top
Master | Next Rank: 500 Posts
Posts: 347
Joined: Mon Aug 04, 2008 1:42 pm
Thanked: 1 times

by Stockmoose16 » Mon Sep 08, 2008 2:41 pm
bourne159 wrote:You got one half of the answer. You forgot the other part.
The question did not say find the probability of first marble being white and the second marble being Blue.

It jut asked for probability of picking up a white marble and a blue marble.

Probability of picking one white and one blue = P(W). P(B) + P (B). P(W)
2/10.5/9 + 5/10.2/9 = 1/9 + 1/9 = 2/9
Huh? What is the difference between "The probability of first marble being white and second being blue" and "Picking up a white and blue marble"? Those are the exact same questions in my mind. Both times, you're picking up two marbles. You want exactly one white. Then exactly one blue. What am I missing?
Top
Senior | Next Rank: 100 Posts
Posts: 50
Joined: Tue Mar 11, 2008 6:30 am
Thanked: 6 times

by bourne159 » Mon Sep 08, 2008 2:58 pm
Stockmoose,

In probability the order is important. When asked what is the probability of getting a white marble and blue marble you have to find both
a) probability of first getting a white ball and then a blue ball +
b) probability of first getting a blue ball and then a white ball
Top
Master | Next Rank: 500 Posts
Posts: 347
Joined: Mon Aug 04, 2008 1:42 pm
Thanked: 1 times

by Stockmoose16 » Mon Sep 08, 2008 3:04 pm
bourne159 wrote:Stockmoose,

In probability the order is important. When asked what is the probability of getting a white marble and blue marble you have to find both
a) probability of first getting a white ball and then a blue ball +
b) probability of first getting a blue ball and then a white ball
Bourne,

What's the difference between "What is the probability of getting a white marble and blue marble" and "What is the probability of first picking a white marble, then picking a blue one? Can you explain how these are logically any different?
Top
Master | Next Rank: 500 Posts
Posts: 347
Joined: Mon Aug 04, 2008 1:42 pm
Thanked: 1 times

by Stockmoose16 » Tue Sep 09, 2008 2:24 pm
Stockmoose16 wrote:
bourne159 wrote:Stockmoose,

In probability the order is important. When asked what is the probability of getting a white marble and blue marble you have to find both
a) probability of first getting a white ball and then a blue ball +
b) probability of first getting a blue ball and then a white ball
Bourne,

What's the difference between "What is the probability of getting a white marble and blue marble" and "What is the probability of first picking a white marble, then picking a blue one? Can you explain how these are logically any different?
Bump. Somebody PLEASE help. I do not understand the logic here. I'm so confused.
Top
Legendary Member
Posts: 2467
Joined: Thu Aug 28, 2008 6:14 pm
Thanked: 331 times
Followed by:11 members

by cramya » Tue Sep 09, 2008 7:56 pm
Case 1:

In the question it says picking 2 marbles.

This means you could picked the white marble first and then the blue marble (ACHIEVED YOU END GOAL)

(or)

Pick the blue marble first and then the white marble (ACHIEVED YOUR END GOAL)

When its an OR you add the probablities


Case2:
If the problem reads "What is the probability of first picking a white marble, then picking a blue one"

IMO Then its simply 2/10*5/9

The OR situation does not happen since you have to pick the white marble first
Top
Post new topic Post Reply
• Page 1 of 1