Addition to a thousand
You learn to add and subtract to 1000 in round steps, so you never work with awkward numbers all at once.
How to work it out
- If numbers are round, drop the zero: 460 + 80 becomes 46 + 8, then put the zero back.
- If calculating with a whole hundred, only the hundreds digit changes.
- Otherwise: do tens first, then ones.
A worked example
460 + 80
- Both numbers are round, so leave off the 0: 46 + 8 = 54.
- Put the 0 back.
- So 460 + 80 = 540.
More examples
563 − 200
- Only the hundreds change: 500 − 200 = 300.
- The 63 stays the same: 300 + 63 = 363.
875 + 25
- First add the tens: 875 + 20 = 895.
- Then add the ones: 895 + 5 = 900.
Try it yourself
390 + 20 =
630 + 80 =
Watch out
You forget to put the 0 back: for 460 + 80 you say 54.
Look at how big the numbers are. An answer of 54 cannot be right when you start with 460.
Tips
- A round number works like a small number: 460 + 80 is like 46 + 8.
- Partition a number like 25 into 20 and 5, and you have two easy steps.
Exceptions
- If a jump crosses the hundred, like 875 + 25, it helps to jump to 900 first.
Why it works
Round numbers work just like small numbers: 460 + 80 is like 46 + 8, with a 0 on the end. When you add or take away whole hundreds, only the hundreds digit changes. And you can do a number like 25 in two steps: first 20, then 5.