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?

Answer 1

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

Answer 2

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