Grade 2 · Place Value · MAT-02-PV-006
Mentally Add or Subtract 10 or 100
Mentally add or subtract 10 from numbers within 1000.
Learning objectives
What you should be able to do
- Mentally add or subtract 10 from numbers within 1000.
- Mentally add or subtract 100 from numbers within 1000.
- Explain which place values change and when regrouping occurs.
Prerequisite check
Make sure the foundation is ready.
Checks the place-value structure needed for mental jumps.
Learn
Build the idea from meaning, not memorization.
Adding 10 changes one ten
Adding 10 means adding one group of ten, not one single unit. In 243+10, the ones remain 3 while the tens increase from 4 tens to 5 tens, producing 253.
Subtracting 10 removes one ten
Subtracting 10 lowers the tens value by one group of ten. If a boundary is crossed, regrouping affects the hundreds place, as in 304−10=294.
Adding or subtracting 100 changes hundreds
A jump of 100 adds or removes one hundred. Tens and ones usually remain unchanged because 100 contains zero tens and zero ones.
Boundary cases reveal regrouping
Numbers such as 290+10 or 300−10 show why place value matters. The operation changes one ten, but regrouping can change two written digits while the numerical change is still exactly 10.
Interactive math lab
Change it. See what stays true.
Number Line Explorer
Move along the number line.
4 is one less. 5 and 6 is one more.
- jump plus 10
- jump minus 10
- jump plus 100
- jump minus 100
Worked examples
Twenty different ways to see the concept work.
243+10
253Add one ten.
243−10
233Remove one ten.
243+100
343Add one hundred.
243−100
143Remove one hundred.
290+10
300Ten more crosses a hundred boundary.
300−10
290Ten less requires regrouping one hundred into tens.
905+10
915Ones stay 5; tens increase.
905−10
895Cross the 900 boundary correctly.
680+100
780Only hundreds increase.
680−100
580Only hundreds decrease.
999−100
899Remove one hundred.
899+100
999Add one hundred.
470+10
480Add one ten.
470−10
460Subtract one ten.
990+10
1000A ten jump reaches one thousand.
1000−100
900Remove one hundred.
730−100
630Hundreds decrease by one.
730+10
740Tens increase by one.
506−10
496Regroup across the hundreds/tens boundary.
506+100
606Hundreds increase while 06 remains.
Practice
Guided practice with hints
01347+10
Hint: Change tens.
Answer: 357
02347−100
Hint: Change hundreds.
Answer: 247
03600−10
Hint: Think 60 tens minus 1 ten.
Answer: 590
04890+10
Hint: Cross the hundred boundary.
Answer: 900
05408+100
Hint: Keep 08 and add one hundred.
Answer: 508
061000−10
Hint: One ten less.
Answer: 990
Practice
Independent practice
01521+10
Answer: 531
02521−10
Answer: 511
03521+100
Answer: 621
04521−100
Answer: 421
05390+10
Answer: 400
06400−10
Answer: 390
07875+100
Answer: 975
08975−100
Answer: 875
Common mistakes
Learn to catch the error, not just the answer.
Think one ten, not one one.
100 adds one hundred; tens and ones remain unless a larger boundary is crossed.
Trade one hundred for ten tens, then remove one ten.
Applications & challenge
Use the idea beyond a single exercise.
Applications
- Mentally adjust prices or scores by 10 or 100.
- Use a number line to model elevator-like jumps of 10 or 100.
Challenge problems
Why is 299+10=309 rather than 2109?
Add one ten: 29 tens +1 ten =30 tens =300, with 9 ones.
What is 10 less than 1000?
990
What is 100 more than 905?
1005
Explain why 408+100 leaves the 08 unchanged.
The added 100 has zero tens and zero ones.
Flashcards
Retrieve it from memory.
Flashcards
Card 1 of 6
Game mode
Place-Value Jump
Reach target numbers using only ±10 and ±100 moves.
Practice
Mastery check
01456+10
Answer: 466
02456−100
Answer: 356
03700−10
Answer: 690
04990+10
Answer: 1000
05605+100
Answer: 705
06605−10
Answer: 595
07What place changes with +100?
Answer: The hundreds place, barring larger regrouping.
08Explain 300−10.
Answer: Regroup one hundred as ten tens, remove one ten, leaving 290.
Terminology
Words to know
- mental math
- Solving without writing every calculation step.
- jump
- A fixed change on a number line or sequence.
- regroup
- Exchange equivalent place-value units.
- difference
- The result of subtraction.
Continue learning
Connected concepts
Curriculum references
Standards and instructional references
California Common Core State Standards for Mathematics — 2.NBT.B.8
Mental addition and subtraction of 10 or 100.