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PERMUTATION = Arrangement (Order matters). COMBINATION = Selection (Order doesn't matter).
Ask: Does order matter? Yes → P. No → C.
Formula:
ⁿPᵣ = n!/(n-r)! — Order MATTERS
Example:
Arrange 3 out of 5 books.
⁵P₃ = 5!/2! = 120/2 = 60 ways