Posted by **Katia** on Monday, December 13, 2010 at 8:17pm.

my problem is:

From the box containing 5 red balls, 7 blue balls, 10 black balls. you select 4 balls

what is the probabilty that EXACTLY 2 balls are black.

i know that you start it/set it up like this:

c(10,2)c(2,1) divided by c(22,4)

and i can get the answer by plugging it in the calculator. but i would like to know how to solve it by hand. help?

- Math -
**Reiny**, Monday, December 13, 2010 at 10:02pm
I answered it for you here

http://www.jiskha.com/display.cgi?id=1292286864

to evaluate without a calculator ? Wow

C(10,2) = 10!/(8!2!) = 10*9/2 = 45

C(12,2) = 12!/(10!2!) = 12*11/2 = 66

C(22,4) = 22!/(18!4!) = 22*21*20*19/(4*3*2*1) = 7315

You sure you want to do this "by hand" ?

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