Year 3

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We can add and subtract one–digit and two–digit numbers in our heads.

Year 3 Unit 4

What we are learning:

  • Addition can be done in any order. The order chosen should reflect the numbers in the addition:
    With 23 + 8 start with the 23 (biggest number) and add on 8
    With 7 + 45 start with 45 (biggest number) and add on 7
    With 5 + 17 + 5 add the 5s first to make 10, then add 17
    Always look for the simplest order to add the numbers together.
  • Subtraction can only be done in one order. The method used for mental subtraction should be chosen to reflect the numbers in the subtraction:
    With 24 – 3 start on 24 and count back 3 (we are not taking many away)
    With 24 – 21 start on 21 and count on until we get to 24 (difference between the number is small)
  • Numbers can be partitioned (broken down) into smaller parts but still retain the same value. 7 is the same as 3 + 4, 2+2+2+1 or 5+2 etc, and when adding/subtracting mentally it is important to consider the best way to partition the number in order to make the calculation as easy as possible. For example to calculate 18+7, we can partition the 7 into 2+5 to make our calculation easier, we then solve 18+2 (=20) + 5 (=25).
  • When adding several one-digit numbers together look for pairs that add to make 10 (number bonds to 10) and add these first. 

Activities you can do at home:

Activity 1 Finding quick ways to add up
Write down 5 random one-digit numbers. Add them quickly together by:
Looking for any pairs (or combinations of 3 numbers) that add to 10, add these first, start a running total.
Looking for any doubles (i.e. 1,1 ,5,6,3) add these together next and add to the running total.
Look for any numbers that you can add to the running total to make a multiple of 10 (i.e. running total 13, numbers left to add on 5 and 7. Add 7 on next as 13 +7 = 20)
Add any remaining numbers to the running total.

Activity 2 Quick mental addition
When you have to add a sequence of numbers, always look for the pairs or combinations that you know the answer to first, e.g. number bonds to 10 and 20. This will make the rest of the task faster and easier.

Activity 3 Building target numbers
Write 10 in the middle of a piece of paper and draw a circle around it. Ask your child to write down as many different ways as they can of adding 3 numbers together to make 10. This exercise will support your child to look for and recognize combinations that total 10. Extend this exercise by changing it to 4 numbers that add to 10, or 5 numbers that add to 20!

Activity 4 Quick counting
Write down the numbers 1 to 9 in a horizontal line on a piece of paper. Give your child some time to work out the total of all the numbers. When they have calculated the total, explore together the method of adding (in pairs)
numbers from opposite ends of the line:
1 2 3 4 5 6 7 8 9
Begin with 1 and 9, then 2 and 8, then 3 and 7 … show your child that we end up with 4 pairs that total 10 (=40) and another 5 (=45). Always look for the easiest methods.

Good questions to ask:

What is the sum of these numbers?
Can we add these in any order?
Which order makes the sum easiest for you?
What is the difference between these two numbers?
Which number do we take away from which?
How many different ways can we make the number ….. by adding or subtracting?
How might you partition that number to help you?

If your child:

Has a preferred method for adding or subtracting two-digit numbers that works
Let them use their method, but show them other choices if they are faster

Extension Activity

Please use this activity when you think your child understands the unit of work. It will deepen and extend your child’s understanding of this unit.


Extension activity (PDF)