Combinations versus permutations
neelems
can anyone explain when or which types of problems require combinations and when or which problems require permutations?
martha2A: Feb. 4, 2015, 6:40 p.m.
Combination is used when you don't care about order. An example would be if you were asked how many possible ways 5 people could be chosen out of 30. This would be (30 choose 5) Permutation is used when you do care about the order. For example, if you were asked how many possible ways 5 people could be chosen out of 30 if we care about the order they are selected in.
weisbart: Feb. 5, 2015, 7:44 p.m.
Yes. Remember that to compute combinations we calculate the permutations and then divide by the number of orderings to get rid of them. The example martha2A gives is a good one.
iraianne: Feb. 7, 2015, 11:35 p.m.
so for the example Martha2A gave. The permutation would be (30*29*28*27*26) correct?
Soniakumr: Feb. 8, 2015, 3:14 p.m.
That is correct and for the combination, when order doesn't matter, it would be (30*29*28*27*26)/5! to take away the orderings which is the same thing as (30 choose 5).