A known addition fact can grow
If 3 + 4 = 7, then 30 + 40 = 70.
The numbers are ten times larger, so the answer is also ten times larger.
Year 2
Recall and use addition and subtraction facts to 20 fluently, and derive and use related facts up to 100
Use secure facts within 20, then link them to related facts with larger numbers up to 100.
Lesson overview
Number - addition and subtraction
When a fact within 20 is secure, it can help with a related fact up to 100. If 3 + 4 = 7, then 30 + 40 = 70. If 15 - 6 = 9, then 19 - 10 = 9 also keeps the same answer. Known facts help us think more quickly and clearly.
A subtraction fact can stay connected
If 15 - 5 = 10, then 50 - 40 = 10.
Both number sentences are linked by the same target answer of 10.
A bond to 10 helps with 100
If 6 and 4 make 10, then 60 and 40 make 100.
Knowing the bond to 10 helps you see the matching bond to 100.
Worked example
Use the fact 8 + 2 = 10 to help with 80 + 20.
- Start with the known fact 8 + 2 = 10.
- Now think about the related tens fact.
- 80 is 8 tens and 20 is 2 tens.
- 8 tens and 2 tens make 10 tens.
- 10 tens make 100, so 80 + 20 = 100.
Try it
Look at the fact, then choose the related fact or answer that matches it.
Question 1
Question 2
Question 3
Question 4
Question 5
Common mistakes
Watch for these when you link a known fact to a larger related fact.
- Forgetting that the linked fact changes the size of all the numbers, not just one of them.
- Using a known fact that does not actually match the larger number sentence.
- Mixing up addition and subtraction facts when looking for a related fact.
Related topics
These ideas fit closely with secure addition and subtraction facts.
- Use place value and number facts to solve problems.
- Add and subtract numbers using concrete objects, pictorial representations, and mentally.
- Recognise and use the inverse relationship between addition and subtraction.