Identify all numbers that belong to each of the following collections of numbers, if there are any.

(a) All prime natural numbers less than 10.
(b) All the natural numbers which are less than 100 and divisible by 10.
(c) All the natural numbers which are less than 1.

Each of the collections in Activity 1 are well-defined collections, that is, given a number, one can determine if that number is in the given collection or not. Such collections are called sets.

Definition

A well-defined collection of objects or individuals is called a set.

By well-defined, we mean that, given an object, we are able to determine whether the object is in the set or not, without any ambiguity.