1. Counting, Permutations, & Combinations

2. A counting problem asks “how many ways” some event can occur.
How many three-letter codes are there using letters A, B, C, and D if no letter can be repeated?
One way to solve is to list all possibilities.

3. ABC
CAD
ABD
CBA
ACB
CDA
ADB
CDB
ADC
CBD
ACD
DAB
BAC
DAC
BAD
DBA
BCA
DCA
BDA
DBC
BCD
DCB
BDC
CAB
24 Ways

4. An experimental psychologist uses a sequence of two food rewards in an experiment regarding animal behavior. These two rewards are of three different varieties. How many different sequences of rewards are there if each variety can be used only once in each sequence?
Next slide

5. Another way to solve is a factor tree where the number of end branches is your answer.

6. Fundamental Counting Principle
Suppose that a certain procedure P can be broken into n successive ordered stages, S1, S2, Sn, and suppose that
S1 can occur in r1 ways.
S2 can occur in r2 ways.
Sn can occur in rn ways.
Then the number of ways P can occur is

7. Ex. 2: An experimental psychologist uses a sequence of two food rewards in an experiment regarding animal behavior. These two rewards are of three different varieties. How many different sequences of rewards are there if each variety can be used only once in each sequence?
Using the fundamental counting principle: 3 2

8. Permutations
An r-permutation of a set of n elements is an ordered selection of r elements from the set of n elements
! means factorial
Ex. 3! = 3∙2∙1
0! = 1

9. How many three-letter codes are there using letters A, B, C, and D if no letter can be repeated?
Note: The order does matter

10. Combinations
The number of combinations of n elements taken r at a time is
Order does NOT matter!
Where n & r are nonnegative integers & r < n

11. How many committees of three can be selected from four people?
Use A, B, C, and D to represent the people
Note: Does the order matter?

12. How many five-card hands are possible from a standard deck of cards?

13. In how many ways can 9 horses place 1st, 2nd, or 3rd in a race?

14. In how many ways can a team of 9 players be selected if there are 3 pitchers, 2 catchers, 6 in-fielders, and 4 out-fielders?