prob/stats definition a permutation is an ordered arrangement of objects. (for example, consider the...
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Prob/Stats
Definition
A permutation is an ordered arrangement of objects.
(For example, consider the permutations of the letters A, B, C and D.)
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The number of permutations of nn objects is nn!
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For example, consider how many permutations of the letters A, B, C and D we have, if we only select two at a time :
Permutations: {A, B} ; {A, C} ; {A, D} ; {B, A} ; {B, C} ; {B, D} ; {C, A} ; {C, B} ; {C, D} ; {D, A} ; {D, B} ; {D, C}
P(4, 2) = 12
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PERMUTATIONS OF n OBJECTS TAKEN r AT A TIME:
P(n, r) = n!
(n – r )!
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Example 1:
A club has 9 members. In how many ways can a president, vice-president, and secretary be chosen from the members of the club?
Answer: 504
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Example 2:
A horse race has 10 horses entered. The 1st , 2nd, and 3rd place finishers will be announced. How many outcomes are there?
Answer: 720
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Definition
A combination is an arrangement of objects where the order does not matter.
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Again, consider how many combinations of the letters A, B, C and D we have, if we only select two at a time :
Combinations: {A, B} ; {A, C} ; {A, D} ; {B, C} ; {B, D} ; {C, D}
C(4, 2) = 6
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COMBINATIONS OF n OBJECTS TAKEN r AT A TIME:
C(n, r ) = n!
r ! (n – r)!
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Example 3:
A club has 9 members. How many ways can a committee of 3 be chosen from the members of the club?
Answer: 84
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Example 4:
To win the UK National Lottery (in 1995) you had to correctly choose 6 different numbers, each being between 1 and 49. What is the probability of winning?
Answer: 1 in 13,983,816
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Guidelines for Solving Counting Problems :
If the order matters, then use permutations.
If the order does not matter, use combinations.
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Exercise 1: A poker hand consists of 5 cards randomly dealt from a standard 52-card deck. How many different poker hands are possible?
Answer: 2,598,960
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Exercise 2: A hat contains 10 raffle tickets. Four tickets are to be selected. The holder of the 1st ticket wins a car, the 2nd wins a motorcycle, the 3rd wins a bicycle, and the 4th wins a wakeboard. How many different ways can these prizes be awarded?
Answer: 5040
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Permutation OR Combination?
How many ways can you choose 4 books from a section of 7 different books?
How many ways can 4 books be arranged on a shelf from a selection of 7 different books?
How many batting lineups can you make for a 9-man baseball team?
How many basketball starting lineups can you create from a 12-man roster?
CombinationCombination
PermutationPermutation
PermutationPermutation
CombinationCombination