Learn in the textbookProbability and Counting (Number of Cases)
DefinitionA combination (組合せ) count is the total number of ways to choose items from several distinct ones without regard to order.
ExplanationThe number of combinations of r items chosen from n is written nCr and is found by dividing the permutation count nPr by r!, the number of ways to order the r chosen items. Choosing r items is the same as choosing the n − r items left behind, so nCr = nC(n−r) holds.
AnalogyIt is like picking three ingredients for a hot pot. Whatever order you add them in, the same ingredients make the same pot.
Memory hookThe 組 in 組合せ means a team or group, and a group has no order inside it.