I keep getting “Permutation” questions wrong in Permutations & Combinations. What's the trap?
using permutations when order doesn't matter.
The usual trap: using permutations when order doesn't matter. How to avoid it: if rearranging counts as different, it's a permutation. Go back to the definition — an arrangement where order matters: nPr = n!/(n−r)!.
Key point
Permutation
Common mistake
using permutations when order doesn't matter.
Memory tip
if rearranging counts as different, it's a permutation.
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