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)
1 · Read
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.
More to read
Add and subtract within 100: FAQ
Tens hops move only the tens digit on a clear road. For 345 plus 40, the 4 tens grow to 8 tens while 305 waits calmly, landing at 385. Cover the hundreds and ones with sticky notes and watch the tens digit climb alone. Uncover to confirm the rest never budged.
Multi-Digit Addition with Regrouping
Taking tens away mirrors adding them. For 462 take away 30, the 6 tens shrink to 3 tens and the answer lands at 432. Slide 3 ten rods off your block tower and read the new height. The hundreds and ones stand still like goalposts while the tens do the running.
Subtracting Numbers with Regrouping Using Models
Bridges are the fun exception to the tens-only rule. In 275 plus 80, the tens overflow past 10 tens and hand a fresh hundred left, landing at 355. Spot the overflow coming: if the tens pile would pass 9, warn the hundreds that a guest is arriving. Predict the bridge, then compute to confirm.
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Where it sits
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.