# drawing marbles from a bag

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• Sep 22nd 2008, 03:31 AM
addison
drawing marbles from a bag
if there are 20 white marbles and 32 black marbles in a bag
what is the probability of drawing 20 marbles where 10 are white and 10 are black ?
is there a formula ?
• Sep 22nd 2008, 03:46 AM
mr fantastic
Quote:

Originally Posted by addison
if there are 20 white marbles and 32 black marbles in a bag
what is the probability of drawing 20 marbles where 10 are white and 10 are black ?
is there a formula ?

Yes. Read this: Hypergeometric distribution - Wikipedia, the free encyclopedia.

In your case: N = 52, n = 20, m = 20, k = 10.
• Sep 22nd 2008, 10:14 AM
Soroban
Hello, addison!

Quote:

There are 20 white marbles and 32 black marbles in a bag.
What is the probability of drawing 20 marbles where 10 are white and 10 are black?

There are: . ${52\choose20}$ possible outcomes.

To draw 10 white marbles, there are: . ${20\choose10}$ ways.

To draw 10 black marbles, there are: . ${32\choose10}$ ways.

. . Hence, there are:\ . ${20\choose10}{32\choose10}$ ways to get 10 white and 10 black marbles.

Therefore: . $\boxed{P(\text{10 white, 10 black}) \;=\;\frac{{20\choose10}{32\choose10}}{{52\choose2 0}}}$