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
The lesson
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.
Watch it
Where it sits
This opens up
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.