Mentally adding hundreds to 3-digit numbers
Mentally add and subtract a three-digit number and hundreds
What a learner can do afterwards
- Calculate 345 + 200 mentally
- Calculate 762 − 400 mentally
- Explain that only the hundreds digit changes
The lesson
You already know how to add or take away 10 or 100 from a number. This works the same way with a whole three-digit number. When you add or subtract hundreds, only the hundreds digit changes. The tens digit and the ones digit stay exactly the same.
Ben has 762 marbles. He gives away 400 marbles. The tens digit (6) and the ones digit (2) do not change. Only the hundreds digit changes: 7 hundreds minus 4 hundreds is 3 hundreds. Ben has 362 marbles left.
Cover the tens and ones digits with your finger. Add or subtract just the hundreds, like a one-digit number. Then put the tens and ones back where they were.
Adding or subtracting hundreds only changes the hundreds digit; the tens and ones stay exactly the same.
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.