Mentally adding and subtracting ones with 3-digit numbers
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)
1 · Read
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.
More to read
Add and subtract within 1,000: FAQ
Mental maths splits numbers into friendly piles. For 345 plus 7, park the 300 and wrestle only with 45 plus 7, which is 52. Glue the parked part back: 300 plus 52 is 352. The hundreds stayed put, while the tens moved up by one because 5 plus 7 made a new ten.
Add and subtract within 1,000: FAQ
Taking small bites works by nibbling the ones first. For 462 take away 5, park the 400 and solve 62 take away 5, but even easier: 2 take away 5 needs a ten break, landing at 457. Say the parked part and the nibbled part together to check the landing spot.
Adding and subtracting within 1,000: FAQ
Watch which digits move and you will predict answers before computing. Adding 7 to 345 touched the ones and the tens, because 5 plus 7 crossed a ten boundary. Near a boundary like 500 take away 6, the tens and ones tumble too, landing at 494. Name the moving digits first, then calculate to confirm.
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2 · Watch
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Where it sits
Optional connections
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Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
17 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.