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Natural numbers are the simplest type of numbers we learn as children. These numbers start from 1 and go on infinitely (1, 2, 3, 4, 5, ...).
They are positive integers and do not include zero or any negative numbers. Natural numbers are used for counting and ordering in everyday life. For example:

• Counting objects: 1 apple, 2 apples, 3 apples
• Ordering: 1st place, 2nd place, 3rd place

Understanding natural numbers is crucial because it helps in grasping more complex mathematical concepts later on.

Multiples are the result of multiplying a number by an integer. For example, the multiples of 5 are obtained by multiplying 5 by 1, 2, 3, and so on.
More generally, the multiples of a number 'n' can be written as:

• n, 2n, 3n, ...

In simpler terms, you can think of multiples as the 'skip counting' pattern of a number. For example, the multiples of 5 are 5, 10, 15, 20, 25, etc.
They continue infinitely because there is no end to how many times you can multiply 5 by any positive integer. Knowing how to find multiples helps in solving various mathematical problems like finding common multiples, least common multiple (LCM), and more.

Set notation is a way to clearly define and represent a collection of items, known as a set. These items are called elements of the set. In mathematics, set notation can be of various types, but the two most common are:

• Roster notation
• Builder notation

In roster notation, elements of a set are listed within curly braces \(\text{\textbraceleft} ... \text{\textbraceright}\).
For example, the set of all natural numbers that are multiples of 5 in roster notation is written as: \[ \text{\textbraceleft 5, 10, 15, 20, 25, ...\textbraceright} \] Understanding set notation is important because it provides a systematic way to describe collections of objects. It is commonly used in various branches of mathematics, including algebra, calculus, and statistics.