Methods of Counting By Dr. Carol A. Marinas

Fundamental Counting Principle Event M can occur in m ways Event N can occur in n ways The event M followed by N is m*n ways If Event M is flipping a coin, there are 2 outcomes. If Event N is rolling a die, there are 6 outcomes. So the number of outcomes of flipping a coin and rolling a die is 2 * 6 or 12 ways. Can you list them?

Permutations of Unlike Objects How many ways can you choose 2 marbles from the 5 different colored marbles above if order matters? (G, R), (G,B), (G, W), (G, Y) (R, G), (R, B), (R, W), (R, Y) (B, G), (B,R), (B, W), (B, Y) (W, G), (W, R), (W, B), (W, Y) (Y, G), (Y, R), (Y, B), (Y, W)

Permutation Formula An arrangement of things in a definite order with no repetition. n P r = n ! (n - r)! 5 P 2 = 5 ! (5 - 2)! = 5 * 4 ways

Combinations of Unlike Objects How many ways can you choose 2 marbles from the 5 different colored marbles above if order does not matter? (G, R), (G,B), (G, W), (G, Y) (R, G), (R, B), (R, W), (R, Y) (B, G), (B,R), (B, W), (B, Y) (W, G), (W, R), (W, B), (W, Y) (Y, G), (Y, R), (Y, B), (Y, W)

Combination Formula An arrangement of things in which order does not matter with no repetition. n C r = n ! r! (n - r)! 5 C 2 = 5 ! 2! (5 - 2)! = (5 * 4)/2 ways or 10 ways

Permutation of Like Objects Before the objects were distinctly different, what if some objects were alike? How many ways can you arrange the letters in “tot”? tot, tto, ott 3 ways

Permutation of Like Objects How many ways can you arrange the letters in “Mississippi”? This would be a lot of work to list. There must be an easier way! Formula n ! r1! r2! r3! … rk! For “tot”, it is 3 ! = 3 ways 2! 1!

Permutation of Like Objects How many ways can you arrange the letters in “Mississippi”? Formula n ! r1! r2! r3! … rk! For “Mississippi”, it is 11 ! = ways 1! 4! 4! 2! 11 letters so n = 11 1 M, 4 I’s, 4 S’s, and 2 P’s

Thanks for Counting with me!