Add ones
246 + 3 = 249.
Only the ones change here.
Year 3
Add and subtract numbers mentally, including a three-digit number and ones, tens and hundreds
Add and subtract ones, tens and hundreds mentally by using place value.
Lesson overview
Number - addition and subtraction
Mental calculation works best when we notice which part of the number changes. Adding ones changes the ones. Adding tens changes the tens. Adding hundreds changes the hundreds.
Add tens
246 + 20 = 266.
The tens increase by 2 tens.
Subtract hundreds
746 - 300 = 446.
The hundreds change and the tens and ones stay the same.
Worked example
Work out 532 - 40 mentally.
- Look at the tens in 532.
- Subtracting 40 means taking away 4 tens.
- 53 tens and 2 ones become 49 tens and 2 ones.
- So 532 - 40 = 492.
Try it
Decide whether ones, tens or hundreds are changing before you answer.
Question 1
256 + 4 =
Question 2
431 + 30 = ?
Question 3
620 - 200 = ?
Question 4
708 - 6 =
Question 5
Which answer is correct for 390 + 100?
Common mistakes
Watch for these when working through the lesson.
- Changing the wrong digit in the number.
- Treating 20 like 2.
- Forgetting to cross a boundary carefully when subtracting.
Related topics
These ideas fit closely with this lesson.
- Add and subtract numbers with up to three digits using formal written methods of columnar addition and subtraction.
- Estimate the answer to a calculation and use inverse operations to check answers.