book

Year 3 Maths

maths owl

Doubles and Halves

doubles and halves

When you add a number to itself, you are doubling the number, or multiplying by two. When you add 3 and 3, you double 3. 3 + 3 = 6, or 3 × 2 = 6. So double 3 is 6. Another way to say that is ‘twice 3 is 6’. Practise doubling the numbers from 1 to 9 until you know them by heart. On a sheet of notepaper, write out the sums of doubles: 1 + 1 = ___, 2 + 2 = ___ ... Fill in the answers and complete the rest to the sum of 9 + 9 = __.

In What Your Year 2 Child Needs to Know, we started learning the multiplication tables for 2, 5 and 10. What is 1 × 2? Can you find the products of each number from 1 to 9 multiplied by 2? On your sheet of notepaper, write out the products of doubles: 1 × 2 = __, 2 × 2 = __ ... Fill in these answers and complete the rest up to 9 × 2 = __.

Now look at the sums of the doubles you found, and the products of each number multiplied by two. Do you see a pattern? Do you see how the answers go up by twos? Did you also notice that when you double any number, the result is always an even number?

Even if you double an odd number, the double turns out even. Try it: double 3 marbles, and you get 6 marbles. Double 5 and you get 10. So, all even numbers are a number doubled. What number is doubled to get 2? What number is doubled to get 18?

Doubles can help you when you're adding. If you know that 7 + 7 = 14, then you can quickly work out the sum of 8 + 7. You know that 8 is one more than 7. So, 8 + 7 must be one more than 7 + 7.

7 + 8 is the same as 7 + 7 + 1, or (7 × 2) + 1
7 + 8 = 14 + 1
7 + 8 = 15

On your sheet of notepaper, try these doubles-plus-one problems on your own in the same way that we did the last one:
5 + 6 = __
8 + 9 = __
4 + 5 = __

If you learn your doubles up to 20, then you'll also know how to divide numbers in half. If you cut a piece of toast into two equal parts, then each part is a half. When a number is divided into two equal parts, each part is a half. What is half of 8? You know the answer if you know what number you double to make 8. You double 4 to make 8. So, 4 is half of 8.

What number do you double to get 4? To get 4, you double 2. So, half of 4 is 2. What number do you double to make 12? To get 12, you double 6. So, half of 12 is 6. Can you tell me what half of 6 is?


This activity is adapted from pages 235 - 236 of What Your Year 3 Child Needs to Know, which can be purchased here.