This page explores the numbers between 0 and 20 and the basic arithmetic of adding and subtracting these numbers.
It is equivalent to Key Stage 1 (years 1 and 2 of schooling in England and Wales) and Functional Skills Entry Level 1.
The First 20 Numbers
You should be able to read, write, count and compare the first 20 numbers of the number system.
For reference, the numbers 1 to 20 are given below in both written and numerical form:
| Number | Name |
| 0 | Zero |
| 1 | One |
| 2 | Two |
| 3 | Three |
| 4 | Four |
| 5 | Five |
| 6 | Six |
| 7 | Seven |
| 8 | Eight |
| 9 | Nine |
| 10 | Ten |
| 11 | Eleven |
| 12 | Twelve |
| 13 | Thirteen |
| 14 | Fourteen |
| 15 | Fifteen |
| 16 | Sixteen |
| 17 | Seventeen |
| 18 | Eighteen |
| 19 | Nineteen |
| 20 | Twenty |
If you do not already know these numbers, it is important that you memorise them and practice reading and writing them in both written and numerical form as well as saying them out loud.
When we count things, we always start at the beginning with one (1). You should practice counting from one (1) to twenty (20).
You should also practice counting backwards from twenty (20) to one (10)
If you are counting items less than twenty (20), you should stop counting when there are no more items left to count. The final number you count is the number of items that there are.
One More Number: 0 (Zero)
There is also one more important number that comes before one (1) and that is zero (0).
Zero can feel a little abstract as it means none or nothing.
Comparing Numbers
The Number Line below shows the numbers 0 to 20 in numerical order.
If a particular number is further to the right of the number line than another number then it is bigger or higher than that number.
For example: 10 is bigger than 5.
If a particular number is further to the left of the number line than another number then it is smaller or lower than that number.
For example: 5 is lower than 10.
The First Mathematical Operators
To be able to understand and represent basic arithmetic on numbers such as adding and subtracting (see below), it is necessary to know the symbols that are universally used.
These symbols are called mathematical operators.
There are several mathematical operators, however at this level we will be looking at just three:
- + (plus)
- – (minus)
- = (equals)
The plus (+) sign is used for adding two numbers together.
The minus (-) sign is used for subtracting one number from another.
The equals (=) sign is used to say that two or more things are equivalent to one another. In basic arithmetic, the answer to a sum is usually placed to the right of the equals sign (with the sum itself on the left).
For example:
- 1 + 1 = 2
- 2 – 1 = 1
- 2 + 2 = 4
Basic Arithmetic
When you combine numbers with mathematical operators, you are able to perform arithmetic.
Addition
Addition uses the plus (+) sign. Words associated with addition sums are plus, add and sum.
To add two numbers together, let’s go back to the number line.
You will need to point to the first number and count on to the right for as many times as indicated by the second number.
For example, to find the answer to:
What is 1 + 1?
Find number 1 on the number line and count 1 to the right. What is the number on the number line? That’s right, it is 2. So:
1 + 1 =2.
Let’s try another:
What is 5 +3?
Find number 5 on the number line, then count 3 to the right. 6, 7, 8. The answer is 8.
5 + 3= 8
Subtraction
Subtraction uses the minus (-) sign. Words associated subtraction are subtract, minus and the difference.
Subtraction is similar to addition in that you find the first number line. However, instead of counting to the right, you count to the left.
For example, find the answer to:
What is 8 – 3?
Find the number 8 on the number line, then count backwards 3 times. 7, 6, 5. The answer is 5. So:
8 – 3 = 5
One more:
What is 12 – 1?
Start with number 12 on the number line and count backwards one. The answer is 11.
12 – 1 = 11
Summary
You should now know:
- How to count to twenty
- Be able to compare and order numbers between zero and twenty
- Add numbers up to twenty
- Subtract numbers up to twenty
- Understand the +, – and = signs