This unit includes the properties of addition and multiplication, negative numbers and fractions.

- CME’s Addition and Multiplication Table Patterns
- David Cox uses properties to justify steps
- James Tanton uses area as a model for multiplication, and uses the model to explain why a negative times a negative is a positive.
- A good problem, originating with PCMI, for adding and subtracting fractions.
- Alternate division algorithms for fractions, via Chris Danielson. Why do they work?

Here’s everything posted under “Arithmetic,” including stuff that didn’t make the cut.

Still looking for:

- A really good way to handle multiplication and division of fractions.

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