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PROBABILITY QUESTION (PLEASE HELP!!!)

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by Stockmoose16 » Tue Sep 09, 2008 2:38 pm
I desperately need someone to explain the difference between these two questions:

In a bag of 10 marbles, there are 6 white, 4 blue:

1) "What is the probability of getting a white marble and blue marble"

2) "What is the probability of first picking a white marble, then picking a blue one?

These two questions sound exactly the same to me, but according to my GMAT book, have two different answers. Can someone explain the difference between the two questions in layman's terms?
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Re: PROBABILITY QUESTION (PLEASE HELP!!!)

by parallel_chase » Tue Sep 09, 2008 3:00 pm
Stockmoose16 wrote:I desperately need someone to explain the difference between these two questions:

In a bag of 10 marbles, there are 6 white, 4 blue:

1) "What is the probability of getting a white marble and blue marble"

2) "What is the probability of first picking a white marble, then picking a blue one?

These two questions sound exactly the same to me, but according to my GMAT book, have two different answers. Can someone explain the difference between the two questions in layman's terms?
"What is the probability of getting a white marble and blue marble"

= 6/10 * 4/9 + 4/10 * 6/9

= 4/15 + 4/15 = 8/15 [here order of picking marbles is not mentioned, therefore you have consider both the possibilities]

"What is the probability of first picking a white marble, then picking a blue one?

= 6/10*4/9 = 4/15


Hope this helps.
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Re: PROBABILITY QUESTION (PLEASE HELP!!!)

by Stockmoose16 » Tue Sep 09, 2008 3:02 pm
parallel_chase wrote:
Stockmoose16 wrote:I desperately need someone to explain the difference between these two questions:

In a bag of 10 marbles, there are 6 white, 4 blue:

1) "What is the probability of getting a white marble and blue marble"

2) "What is the probability of first picking a white marble, then picking a blue one?

These two questions sound exactly the same to me, but according to my GMAT book, have two different answers. Can someone explain the difference between the two questions in layman's terms?
"What is the probability of getting a white marble and blue marble"

= 6/10 * 4/9 + 4/10 * 6/9

= 4/15 + 4/15 = 8/15 [here order of picking marbles is not mentioned, therefore you have consider both the possibilities]

"What is the probability of first picking a white marble, then picking a blue one?

= 6/10*4/9 = 4/15


Hope this helps.
Can you please explain why the order makes any difference? (effectively doubling the answer?)
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Re: PROBABILITY QUESTION (PLEASE HELP!!!)

by parallel_chase » Tue Sep 09, 2008 4:03 pm
Stockmoose16 wrote: Can you please explain why the order makes any difference? (effectively doubling the answer?)
The order makes a difference in probability since we are not given any order of picking marbles, like there was in second part of the question.

We are not doubling anything. We are just calculating the total probable combination of picking marbles.
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