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
1 · Read
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.
More to read
Adding and subtracting within 1,000: FAQ
Hundreds hops move only the hundreds digit. For 345 plus 200, the 3 hundreds grow to 5 hundreds while 45 sits out the dance. Glue 45 back on to land at 545. Say the hop aloud: three hundred forty-five, five hundred forty-five. The rhythm proves the small digits never moved.
1.2 Add Whole Numbers
Taking hundreds away mirrors adding them. For 762 take away 400, the 7 hundreds shrink to 3 hundreds while 62 waits aside. Rebuild to land at 362. Blocks show it cleanly: slide 4 hundred flats off the tower and read what is left standing.
Multi-Digit Addition with Regrouping
The hundreds-only rule is a promise you can test. Cover the tens and ones of 625 with your thumb, add 200 to the visible 6, and uncover to find 825 waiting. When hundreds crosses into thousands the rule still holds, just with taller towers. Predict first, uncover second, and trust the hundreds hop.
Explore more
2 · Watch
به انگلیسیTake it off screen
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.