Dividing n distinct objects into groups of sizes r1, r2, ..., rk: n!/(r1! * r2! * ... * rk!). If some groups are of equal size, divide further by the number of equal-sized groups factorial. Example: 12 people into 3 groups of 4: 12!/(4! * 4! * 4! * 3!) — the extra 3! accounts for the groups themselves being indistinguishable if unlabeled.