Finding the Number of Combinations - withOUT regard to ORDER
nCr = n! / (n-r)!r!
Finding the Number of Permutations - when ORDER is important
nPr = n! / (n-r)! where r<=n
The ! is called "Factorial" - it means Multiply by each number ex=4!=4*3*2*1
ex. nCr = 16C4 = 16! / (16-4)!4!
= 16! = 16 *15 * 14 * 13 * 12 * 11 * 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2
12! 4! 4 * 3 * 2 * 12 * 11 * 10 * 9 * 8 * 7 * 6 * 5 * 4 * 3 * 2
= 16! = 16 *15 * 14 * 13 = 43680 = 1820
12! 4! 4 * 3 * 2 24
Formula P(n, r) =
Khan Academy Video on Permutations (Order IS important) https://www.khanacademy.org/math/precalculus/prob-comb/combinatorics-precalc/v/permutation-formula
Khan Academy Video on Combinations (Order NOT important) https://www.khanacademy.org/math/precalculus/prob-comb/combinations/v/introduction-to-combinations