concepts of permutations and combinations, and use only addition, multiplication, and division.
Simple Addition
if there are a varieties of soup and b varieties of salad, then there are a + b possible ways to order a meal of soup or salad ( but not both soup and salad).
a = soup b = salad a + b possible ways to order a meal of soup or salad a or b possible ways
Simple Multiplication
if there are a varieties of soup and b varieties of salad, then there are ab possible ways to order a meal of soup and salad.
a = soup b = salad a * b possible ways to order a meal of soup and salad. a and b possible ways
A permutation of collection of objects is a reordering of them. For example, there are six different permutation of the letter ABC ABC ACB BAC BCA CAB CBA
5 * 4 * 3 * 2 * 1 = 120 different ways to permute the letters in “HARDY”.
n * (n - 1) … 1 is denoted by the symbol , n factorial.