I keep getting “Combination” questions wrong in Permutations & Combinations. What's the trap?
using combinations when order actually matters.
The usual trap: using combinations when order actually matters. How to avoid it: if only the group matters (not its order), it's a combination. Go back to the definition — a selection where order does not matter: nCr = n!/[r!(n−r)!].
Key point
Combination
Common mistake
using combinations when order actually matters.
Memory tip
if only the group matters (not its order), it's a combination.
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