1. Multiplication Rule Combinations Permutations
Counting Problems
Multiplication Rule Combinations Permutations

2. Goals of Lesson Look at 3 special kinds of counting problems
Connect the kind of counting problem to correct formula
Use our calculators to solve counting problems

3. First kind of counting problem: Multiplication Rule
How many different arrangements can be made if
each item can be used more than once for each arrangement

4. Counting Area Codes
The phone company assigns three-letter area codes to represent locations. For example, the area code for Northern California is How many different 3 digit area codes are possible if you can use the same number more than once in an area code?
We will assume that any combination of three numbers is possible.

5. Multiplication Principle of Counting:
(# of choices for 1st position) (# of choices for 2nd position) (# of choices for 3rd position)……
for however many positions
In the area code problem there are 3 positions with 10 choices for each

6. Number of choices for 1st position: 10 Number for 2nd position: 10
How many area codes are possible if we can use a number more than once in the same area code?
Three positions: ____ ____ ____
Available choices for each digit (10 total):
Number of choices for 1st position: 10
Number for 2nd position: 10
Number for 3rd position: 10
Total number of possible area codes
= (10)(10)(10)
= 1000

7. Second kind of counting problem: Permutation
How many different arrangements can be made if
each item can only be used once for each sequence
order matters (a b c not the same as c b a)

8. n Pr P stands for Permutation
Example of a Permutation Problem:
If you can only use each letter once, how many different arrangements can be made using the letters in the alphabet?
(There are 26 letters in the alphabet)
n Pr
P stands for Permutation
n is the number of objects we can pick from
r is the number of objects we use in each arrangement
n = 26 (there are 26 letters) r = 26 because we are randomly choosing 26 at a time

9. How many ways can we arrange the letters of the alphabet if we can only use each letter once? (nPr)
P stands for Permutation
n = 26
r = 26
Calculator Emulator
26 P 26 = 4.03 x 1026

10. Example of another Permutation:
Three members of a club with 20 students will be randomly selected to serve as president, vice president, and treasurer.
The first person selected will be president.
The second person selected will be vice president.
The third person selected will be treasurer.
How many president-vice president-treasurer arrangements are possible?

11. 20 P3 think of it as 20 people picked 3 at a time
How many president-vice president-treasurer combinations are possible?
For these arrangements, we will only use 3 people at a time even though there are 20 people.
Calculator Emulator
20 P3 think of it as 20 people picked 3 at a time
This means: If we have 20 people (number) and we randomly pick 3 at a time, there are 6840 president-vice president-treasurer combinations

12. Betting on the Trifecta
In how many ways can horses in a 10 horse race finish first, second, and third?
Number of horses: 10
Randomly picked 3 at a time
Same horse can’t be 1st and 2nd at same time so no repetition in same pick
Order matters: 1st place isn’t same as 2nd .
10 P 3
Definition of trifecta: a bet where the bettor wins by selecting the first three finishers of a race in the correct order of finish
720 ways that top 3 horses could finish 1st, 2nd, and 3rd

13. Third kind of counting problem: Combinations
How many different arrangements can be made if
each item can be only be used once for each arrangement
order does NOT matter

14. What if order doesn’t matter?
In horse races, order matters
Hedgehunter, Royal Auclair, Simply Gifted placed 1st, 2nd, and 3rd in the 158th Grand National
The order they came in mattered.
In poker, order does NOT matter
if you get 2 kings and 3 queens (full house), it doesn’t matter what order you hold them in your hand)
In poker, the combination of cards matters, not the order.
Horses named above were the 158th Grand National 1st, 2nd, and 3rd place winners.

15. Combinations
Teresa, Bethany, Marissa, and Bridgette are going to play doubles tennis. They will randomly select teams of two players each. List all of the combinations possible.
Teresa-Bethany Teresa-Marissa
Teresa-Bridgette Bethany-Marissa
Bethany-Bridgette Marissa-Bridgette
Order here is not important since Teresa-Bethany is the same team as Bethany-Teresa.

16. Combinations A combination is different from a permutation.
A combination is when order doesn’t matter
In a combination, ABC is the same as BAC

17. Formula for Combinations
n C r
n = number of choices
C stands for Combination
r = number in each arrangement

18. Remember the 4 girls that formed teams of two for tennis?
We wanted to know how many different teams of two could be formed, and order did not matter
n C r
n = 4
r = 2
Calculator Emulator
4 C 2 = 6

19. How many different teams of size 4 can be made if there are 20 people?
20 people  n = 20
randomly choose 4 at a time  r = 4
20 C 4 =
If there are 20 individuals, there are 4845 different groups of 4 that could be formed.

20. What was the first kind of counting problem we looked at today?

21. Multiplication Rule Problems
Arranged a given number of objects
The same object could be used more than once in each arrangement
How many area codes can be formed with the numbers 0 through 9 if numbers can be repeated?
(10) (10) (10) = 1000
We could use a number more than once in an area code. For example: the area code 777 is a possibility. Repetition is allowed.

22. What was the second kind of counting problem we looked at today?

23. Permutation Problems n = 10 r = 3 10 P3 = 720 Arrangement of objects
Could not use the same object more than once in an arrangement
Order mattered (ABC different from BCA)
In how many ways can horses in a 10 horse race finish 1st, 2nd, and 3rd?
n = r = P3 = 720

24. What was the third kind of counting problem we looked at today?

25. n = 4 r = 2 4C2 = 6 Combination Problems
Counting how many ways to arrange objects
Could only use each object once for each arrangement
Order did not matter (ABC same as (BCA)
How many different teams of two can be formed from 4 players?
n = r = C2 = 6

26. If same object can be used more than once in the same arrangement, then Multiplication Rule

27. If same object can NOT be used more than once in the same arrangement, and order matters, then Permutation

28. If same object can NOT be used more than once in the same arrangement, and order DOESN’T matter, then Combination