In doing basic math, you work with many different groups of numbers. The more you know about these groups, the easier they are to understand and work with.

- Natural or counting numbers: 1, 2, 3, 4, …

- Whole numbers: 0, 1, 2, 3, 4, …

- Integers: … –3, –2, –1, 0, 1, 2, 3, …

- Negative integers: … –3, –2, –1

- Positive integers: 1, 2, 3, … (the natural numbers)

*Note:* Zero is neither positive nor negative. It is neutral.

- Odd numbers; integers not divisible by 2:

- … – 5, – 3, – 1, 1, 3, 5, …

- Even numbers: Integers divisible by 2:

- … – 6, – 4, – 2, 2, 4, 6, …

- Rational numbers: Fractions, such as or . All
**integers** are **rational numbers;** for example, the number 5 may be written as . All rational numbers can be written as fractions , with *a* being an integer and *b* being a natural number. Both terminating decimals (such as 0.5) and repeating decimals (such as 0.333 … ) are also rational numbers because they can be written as fractions in this form.

**Irrational numbers:** Numbers that *cannot* be written as fractions , with *a*being an integer and *b* being a natural number. (the Greek letter pi) are examples of irrational numbers.