Mental addition and subtraction (age 7+)
Mentally add and subtract a three-digit number and ones
What a learner can do afterwards
- Calculate 345 + 7 mentally
- Calculate 462 − 5 mentally
- Explain that only the ones digit changes (unless bridging through a ten)
The lesson
A three-digit number is built from hundreds, tens, and ones. When you add or take away a small number, you are really only working with the ones. Most of the time, the hundreds and tens stay exactly the same.
Take 341 + 6. The ones digit is 1, and 1 + 6 = 7. The hundreds and tens do not change at all, so 341 + 6 = 347.
Now look at 462 - 5. The ones digit is 2, and you cannot take 5 away from 2. Borrow a ten from the tens place: 12 - 5 = 7, and the tens digit drops from 6 to 5. So 462 - 5 = 457.
To check if the tens will change, look only at the ones digit. If adding it makes 10 or more, or subtracting it goes below zero, the tens digit changes too.
Check the ones digit first: the tens only change when adding or subtracting crosses ten.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.