One way to denote the number of combinations is C(n,k), but we shall instead use notation that is quite common in probability books: C(n,k) or ([n],[k]), read "n choose k". ([n],[k]) = (m!) / (1) = (n!) / (k!(n-k)!)
Problem 1. Suzy is given a bag containing 4 red marbles, 3 green ones, 2 white ones, and 1 purple one. She grabs five of them. Find the probabilities of the following events, expressing each as a fraction in lowest terms.
4R + 3G + 2W + 1P = 10 marbles in the bag c(10,5) = 252 = n(s).
a) She has all the red ones.
(E) = C(4,4)C(6,1) = 6 (4 red, 1 non-red).
P(E) = (n(E)) / (n(s)) = (6) / (252) = (1) / (42)
b) She has at least one white one.