Place value understanding and number facts
Use place value understanding and number facts to solve problems
What a learner can do afterwards
- Use knowledge that 34 = 30 + 4 to help add 34 + 20 = 54
- Solve 'I think of a number, add 10, and get 45. What was my number?'
- Apply partitioning to solve a problem in an unfamiliar context
The lesson
A two-digit number is made of tens and ones. The number 34 is 3 tens and 4 ones, or 30 + 4. Once you see a number this way, adding gets much faster.
Zoe thinks of a number, adds 10, and gets 45. She knows 45 is 40 + 5. Adding 10 only changes the tens, so her starting number must have been 35, because 35 + 10 = 45.
When you add a whole ten, like 10, 20, or 30, only the tens digit moves. The ones digit stays exactly the same.
Split a number into tens and ones, then add or compare using that instead of counting one by one.
Watch it
Where it sits
This opens up
Nothing builds on it yet.
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.