Addition within a hundred
You learn to add and subtract to 100 by keeping the tens and the ones apart.
How to work it out
- Split both numbers into tens and ones.
- Calculate the tens with tens.
- Calculate the ones with ones.
- Add the two results together.
A worked example
65 + 22
- Tens: 60 + 20 = 80.
- Ones: 5 + 2 = 7.
- Put them together: 80 + 7 = 87.
More examples
76 − 12
- Tens: 70 − 10 = 60.
- Ones: 6 − 2 = 4.
- Put them together: 60 + 4 = 64.
44 + 25
- Tens: 40 + 20 = 60.
- Ones: 4 + 5 = 9.
- Put them together: 60 + 9 = 69.
Try it yourself
23 + 6 =
44 + 25 =
Watch out
You mix up the digits: for 56 + 20 you add the 2 to the 6 and get 58.
Look carefully at which digit is the tens. The 2 in 20 goes with the 50, not with the 6.
When you subtract, you swap the ones round: for 73 − 15 you do 5 − 3.
Always take the second number away from the first. If that does not work, go back to the ten first.
Tips
- Say out loud which digits are tens and which are ones.
Exceptions
- If the ones cross ten, like 48 + 27, you need to bridge through ten.
Why it works
65 is 6 tens and 5 ones. Keep tens with tens and ones with ones, and you only work with small sums you already know. That works for adding and for subtracting.