Adding and subtracting tens mentally
Add and subtract a two-digit number and tens mentally and using concrete/pictorial representations
What a learner can do afterwards
- Calculate 45 + 30 = 75 mentally
- Calculate 82 − 40 = 42 mentally
- Explain that only the tens digit changes when adding/subtracting tens
1 · Read
Adding or subtracting tens is a fast trick. Only the tens digit moves. The ones digit stays exactly where it is.
Now try 82 − 40. Start at 82 and hop back four times by ten: 72, 62, 52, 42. The ones digit stays 2. Only the tens digit changed, from 8 to 4.
Cover the ones digit with your finger. Add or subtract the tens digits only, then uncover the ones digit. It stays the same.
Adding or subtracting tens moves only the tens digit. The ones digit never changes.
More to read
Add and subtract within 100: FAQ
Tens are easy to add in your head. 20 plus 30 is like 2 plus 3, but with tens. 2 tens plus 3 tens is 5 tens. 5 tens is 50. Only the tens digit moves. The ones stay zero. Say the tens aloud to keep track.
Adding and Subtracting Within 100 Using Place Value Strategies (Grade 2)
Subtracting tens works the same way. 82 take away 40 means 8 tens take away 4 tens. That leaves 4 tens. The 2 ones stay. So 82 minus 40 is 42. Picture ten sticks to see why only tens change.
Properties of addition
Explaining the pattern proves you get it. Adding tens changes the tens digit only. The ones digit does not move. For 45 plus 30, 4 tens become 7 tens. The 5 ones stay. So the answer is 75.
Explore more
2 · Watch
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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.