Permutation = ordered arrangement. Combination = unordered selection. Ask yourself: "If I swap two chosen items, does it create a different outcome?" If yes, use permutation. If no, use combination. Forming a 3-digit number from {1,2,3}: 123 differs from 321, so permutation. Choosing 3 players for a team: {A,B,C} = {C,B,A}, so combination. nPr = r! * nCr (every combination can be arranged in r! ways).