If we want to select $r$r item from $n$n items, and the order of the arrangement is important, which of the following should be used?
Combination
Permutation
If we want to select $r$r item from $n$n items, and the order of the arrangement is not important, which of the following should be used?
Calculate the number of permutations of $5$5 objects taken $5$5 at a time.
Does the code used to unlock a combination lock represent a permutation or a combination of objects?