Mentally adding tens to 3-digit numbers
Mentally add and subtract a three-digit number and tens
What a learner can do afterwards
- Calculate 345 + 40 mentally
- Calculate 462 − 30 mentally
- Explain that only the tens digit changes (unless bridging through a hundred)
The lesson
A three-digit number has a hundreds digit, a tens digit, and a ones digit. When you add or subtract tens, like 20, 30, or 40, you are only changing groups of ten. That means the tens digit is the one that moves. The hundreds and ones digits usually stay put.
Take 354 + 40. The hundreds digit is 3 and the ones digit is 4, so those stay the same. The tens digit goes up by 4, from 5 to 9. So 354 + 40 = 394.
Watch what happens when the tens digit gets big. If adding tens pushes it past 9, the extra spills into the hundreds digit. For example, 380 + 40: 8 tens plus 4 tens is 12 tens, which is 1 hundred and 2 tens. So 380 + 40 = 420, and the hundreds digit changes too.
Now try 462 − 30. The ones digit (2) and hundreds digit (4) stay the same. The tens digit drops by 3, from 6 to 3. So 462 − 30 = 432.
Add or subtract only the tens digit, unless the tens spill past 9 or below 0, then the hundreds digit shifts too.
Watch it
Where it sits
This opens up
Nothing builds on it yet.
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.