Learn in the textbookProbability and Counting (Number of Cases)
DefinitionA permutation (順列) count is the total number of ways to pick from several distinct items and arrange them in order.
ExplanationThe number of permutations of r items chosen from n is written nPr and is found by multiplying r numbers that fall by 1 each time, as in n × (n−1) × …. The number of ways to arrange all n items is n × (n−1) × … × 1, called the factorial of n (n!). An arrangement in which the same item can be used any number of times is a permutation with repetition, and with m candidates for each of k positions there are m^k ways.
AnalogyIt is like deciding first, second and third place on a podium. The same three people give a different result whenever their ranks change.
Memory hookThe 列 in 順列 means a line, and once things stand in a line there is a front and a back.