2.NBT.8: Add or Subtract 10 or 100 in Your Head
Also written as 2.NBT.B.8 and CCSS.MATH.CONTENT.2.NBT.B.8 · Cluster: Use place value understanding and properties of operations to add and subtract.
The Official Wording
Mentally add 10 or 100 to a given number 100-900, and mentally subtract 10 or 100 from a given number 100-900.
Common Core State Standards, Grade 2, Number & Operations in Base Ten · Read it at the source · Checked October 14, 2026
What 2.NBT.8 Means in Plain English
Second graders take any number from 100 to 900, say 493, and add or subtract 10 or 100 in their head: 493 + 100 is 593, 493 − 10 is 483. Only one digit moves. The crossing cases, like 493 + 10 = 503, take the most practice.
This is place value made automatic, and it powers the bigger strategies: jumping by hundreds and tens along a number line to solve 356 + 278 (2.NBT.7) starts with these moves. In 3rd grade the same digit-watching turns into rounding to the nearest 10 or 100 (3.NBT.1).
2.NBT.8 I Can Statements
- I can add 10 or 100 to a number in my head.
- I can subtract 10 or 100 from a number in my head.
- I can tell which digit changes and which digits stay the same.
2.NBT.8 Worked Example
One starting number, four mental moves, including the one where the tens roll over into a new hundred.
Start at 493. Find 493 + 10, 493 + 100, 493 − 10 and 493 − 100 without writing anything down.
- Adding 10 touches only the tens digit. 9 tens and one more is 10 tens, a new hundred, so the hundreds go up and the tens become 0. The 3 ones never move.493 + 10 = 503
- Adding 100 touches only the hundreds digit: 4 becomes 5.493 + 100 = 593
- Subtracting 10: the tens digit drops by one, 9 to 8.493 − 10 = 483
- Subtracting 100: the hundreds digit drops by one, 4 to 3.493 − 100 = 393
Check it: Only one digit should change each time, except at a crossing like 493 + 10, where the tens roll over to 0 and the hundreds tick up. Skip-count by tens to confirm: 483, 493, 503.
Common Mistakes with 2.NBT.8
Changing the ones digit
493 + 10 = 494, or 493 + 100 = 494
Try this: Say which digit a ten lives in before moving anything, then cover the ones digit with a finger so it cannot change.
Stuck at the hundred
493 + 10 = 4103, or “there are no more tens”
Try this: Build 493 with 9 rods, add one rod, and trade the ten rods for a flat: 5 flats, 0 rods, 3 cubes is 503.
Counting up by ones
493 + 100 done as a hundred finger counts, or on paper in a column
Try this: Practice on a hundreds chart first, one row down is plus 10, then move to a 1,000 chart or a number line marked in hundreds.
Where 2.NBT.8 Fits
Practice 2.NBT.8
Quick Check
- What is 627 + 100? What is 627 − 10?
- What is 895 + 10?
- A jar holds 340 marbles. Ava adds 100, then takes out 10. How many marbles are in the jar?