# Math-Probability

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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 -

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 -

what does c stand for and how do you get 21?

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