Set theory
Set theory is a way of solving problems in mathematics and logic (reasoning). By studying set theory, you can get a better understanding of arithmetic and of mathematics as a whole.
A set is a collection of objects or ideas. For example, a family, a box of crayons, a flock of sheep, the rules of a game, and the even numbers from 10 to 20 are all sets. The items that make up a set are the members or elements of the set. A red crayon is a member of a set of crayons. The number 16 is a member of the set of even numbers from 10 to 20.
Mathematicians use letters to name sets and the members of sets. Capital letters commonly represent sets, and lower-case letters refer to the members of sets. For example, the letter C might stand for the set of fifth-grade girls with curly hair. The letters m, s, and r might represent the members of this set