Section 4-6 Counting

FUNDAMENTAL COUNTING RULE For a sequence of two events in which the first event can occur m ways and the second event can occur n ways, the events together can occur a total of m · n ways. This generalizes to more than two events.

EXAMPLES How many two letter “words” can be formed if the first letter is one of the vowels a, e, i, o, u and the second letter is a consonant? OVER FIFTY TYPES OF PIZZA! says the sign as you drive up. Inside you discover only the choices “onions, peppers, mushrooms, sausage, anchovies, and meatballs.” Did the advertisement lie? Janet has five different books that she wishes to arrange on her desk. How many different arrangements are possible? Suppose Janet only wants to arrange three of her five books on her desk. How many ways can she do that?

FACTORIALS 𝑛!=𝑛∙ 𝑛−1 ∙ 𝑛−2 ∙⋯∙3∙2∙1 NOTE: 0! is defined to be 1. That is, 0! = 1

FACTORIAL RULE A collection of n distinct objects can be arranged in order n! different ways.

PERMUTATIONS A permutation is an ordered arrangement. A permutation is sometimes called a sequence.

PERMUTATION RULE (WHEN ITEMS ARE ALL DIFFERENT) The number of permutations (or sequences) of r items selected from n available items (without replacement) is denoted by nPr and is given by the formula 𝑛 𝑃 𝑟 = 𝑛! 𝑛−𝑟 !

PERMUATION RULE CONDITIONS We must have a total of n different items available. (This rule does not apply if some items are identical to others.) We must select r of the n items without replacement. We must consider rearrangements of the same items to be different sequences. (The arrangement ABC is the different from the arrangement CBA.)

EXAMPLE Suppose 8 people enter an event in a swim meet. Assuming there are no ties, how many ways could the gold, silver, and bronze prizes be awarded?

PERMUTATION RULE (WHEN SOME ITEMS ARE IDENTICAL TO OTHERS) If there are n items with n1 alike, n2 alike, . . . , nk alike, the number of permutations of all n items is 𝑛! 𝑛 1 ! 𝑛 2 ! ⋯ 𝑛 𝑘 !

EXAMPLE How many different ways can you rearrange the letters of the word “level”?

COMBINATIONS A combination is a selection of objects without regard to order.

COMBINATIONS RULE The number of combinations of r items selected from n different items is denoted by nCr and is given by the formula 𝑛 𝐶 𝑟 = 𝑛! 𝑛−𝑟 !𝑟! NOTE: Sometimes nCr is denoted by 𝑛 𝑟 .

COMBINATIONS RULE CONDITIONS We must have a total of n different items available. We must select r of those items without replacement. We must consider rearrangements of the same items to be the same. (The combination ABC is the same as the combination CBA.)

EXAMPLES From a group of 30 employees, 3 are to be selected to be on a special committee. In how many different ways can the employees be selected? If you play the New York regional lottery where six winning numbers are drawn from 1, 2, 3, . . . , 31, what is the probability that you are a winner? Exercise 34 on page 186.

EXAMPLE Suppose you are dealt two cards from a well-shuffled deck. What is the probability of being dealt an “ace” and a “heart”?

PERMUTATIONS VERSUS COMBINATIONS When different orderings of the same items are to be counted separately, we have a permutation problem, but when different orderings are NOT to be counted separately, we have a combination problem.