Working with numbers
Addition and subtraction
In this unit you add and subtract with numbers up to 100. You use number bonds for 10 to help you, add ones and tens, and choose whether to add or subtract to solve word problems.
Number bonds and complements
Number bonds for 10 are pairs that make 10, like 6 + 4 and 7 + 3. These help you find complements of 20 and of 100. Because 6 + 4 = 10, you also know 16 + 4 = 20 and 60 + 40 = 100.
Adding ones or tens
To add ones to a 2-digit number, add the ones and keep the tens the same: 41 + 6 = 47. To add tens, add to the tens and keep the ones the same: 42 + 10 = 52, and 48 + 20 = 68.
Adding three small numbers
When you add three or more small numbers, look for a pair that makes 10 first. For 4 + 5 + 6, add 4 + 6 = 10 first, then 10 + 5 = 15. Making 10 first makes it easier.
Subtracting ones or tens
To subtract ones, take them from the ones and keep the tens the same: 37 - 5 = 32. To subtract tens, take them from the tens and keep the ones the same: 53 - 10 = 43.
Add or subtract?
For a word problem, decide what is happening. If you are putting amounts together to find how many altogether, you add. If you are taking some away to find how many are left, you subtract.
Worked examples
Use a number bond
- You know 6 + 4 = 10.
- So 16 + 4 = 20 (a complement of 20).
- And 60 + 40 = 100 (a complement of 100).
Answer: 6 + 4 = 10, 16 + 4 = 20, 60 + 40 = 100
Add a ten to a 2-digit number
- Start with 42.
- Add 1 ten: 4 tens + 1 ten = 5 tens.
- The ones stay the same.
Answer: 42 + 10 = 52
Add three small numbers
- Look for a pair that makes 10: 4 + 6 = 10.
- Now add the last number: 10 + 5.
- 10 + 5 = 15.
Answer: 4 + 5 + 6 = 15
Subtract a ten
- Start with 53.
- Take away 1 ten: 5 tens - 1 ten = 4 tens.
- The ones stay the same.
Answer: 53 - 10 = 43
Choose add or subtract
- A bus has 64 seats and 30 people get on.
- You want the empty seats, so take away.
- 64 - 30 = 34.
Answer: There are 34 empty seats.
Where you see this in real life
- Adding a bag of 24 red apples and 10 green apples to find how many Dad bought.
- Taking away pages you have read to find how many are left in a book.
- Working out how many cookies a baker made by adding two batches together.
Key words
- Number bond
- A pair of numbers that add up to make a chosen number, such as 6 + 4 = 10.
- Complement
- The number you add to reach a total like 10, 20 or 100.
- Ones
- The right-hand digit of a number, showing how many single units.
- Tens
- The digit that shows how many groups of ten.
- Total
- The answer when you add numbers together.
- Difference
- The answer when you subtract one number from another.