# drawing marbles from a bag

• Sep 22nd 2008, 02:31 AM
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, 02:46 AM
mr fantastic
Quote:

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, 09:14 AM
Soroban

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: .$\displaystyle {52\choose20}$ possible outcomes.

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

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

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

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