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
1 · Read
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.
More to read
Whole numbers in expanded form review
Every two-digit number is a small sum in disguise. Thirty-four is really 30 plus 4 hiding inside one numeral. Once you spot the hidden sum, adding tens is easy: 34 plus 20 is 30 plus 4 plus 20, and the tens pile grows from 30 to 50. Split the number, grow the matching pile, and glue the parts back together.
Converting Numbers Between Standard and Expanded Form
Undoing an add-10 puzzle runs the same film backwards. If adding 10 landed on 45, the starting number sat one ten lower at 35. Take the ten back off and the mystery number appears. Check by hopping forward again: 35 plus 10 returns to 45. Every add has an undo, and spotting it solves missing-number puzzles fast.
Modeling Numbers in Expanded Form
Partitioning travels well to brand-new puzzles. The same splitting trick that cracks 34 plus 20 also cracks taller numbers and word stories. Break each number into tens and ones, handle the easy piles, then rebuild. When a problem looks strange, ask which piles hide inside it and the path lights up.
Explore more
2 · Watch
En anglaisTake it off screen
Where it sits
Learn first
Optional connections
This opens up
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.