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April 25, 2015

Homework Help: maths

Posted by Candice on Thursday, October 11, 2012 at 9:38am.

A box contains 20 balls, of which 5 are red, 5 are blue, 5 are white and 5 are green. Suppose a sample of 5 balls are chosen without replacement.

a) find the probability that the sample contains balls of all the same colour

(answer is 4/ 20C5, but why isnt it 5/20C5??)

b) find the probability that the sample contains three balls of one colour and two balls of another colour.

The teacher wrote

5C3 x 5C2 x 4P2
---------------
20C5

Could you please explain why this is?
Thanks!!

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