ABSTRACT
This chapter provides two easy rules for enumerating the numbers of elements of certain sets or certain types of arrangements. First, the Addition Principle states that if the author are building an arrangement of objects starting with a set of mutually exclusive beginning states, then the number of arrangements is the sum of the number of arrangements starting from each beginning state. Second, the Multiplication Principle states that if the author are building an arrangement of objects in stages, then the number of arrangements is the product of the number of choices at each stage. Broadly speaking, combinatorics is the branch of mathematics that deals with different ways of selecting objects from a set or arranging objects. The problem was to find a walk through the city that would cross each bridge once and only once, and visit all four parts. The only way to go between the different pieces of land was to cross the bridges.
