Posted by **Emma** on Friday, March 16, 2012 at 11:58pm.

An urn contains four red balls and three white balls. A sample of 2 balls is selected at random from the urn. What is the probability that at least one of the balls is red?

- Math-Probability -
**Reiny**, Saturday, March 17, 2012 at 8:20am
number of ways to pick any two = C(7,2) = 21

number of ways with only white = C(3,2) = 3

so the number of ways with at least one red = 21-3 = 18

prob of at least 1 red = 18/21 = 6/7

- Math-Probability -
**rollerpin**, Wednesday, May 22, 2013 at 11:10am
what does c stand for and how do you get 21?

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