11.3 Counting principles and permutations
Unit 11 · Probability and counting
Ideas in this lesson
Multiplication principle
Multiply the number of choices at consecutive stages when each branch has the stated number of options.
Ordered selections
A permutation counts ordered selections without replacement.
Repeated objects
Divide the total factorial by factorials of indistinguishable repetition counts.
Counting with restrictions
Fix required positions first, then count arrangements of the remaining objects.
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