Ellis showed a subtraction problem, performed both using the traditional regrouping, or borrow-and-carry, algorithm and using an example from the common core. Here’s a screenshot of the two methods.

- Integers are closed for addition and subtraction both. That is, a + b and a – b are again integers, where a and b are any integers. - Addition is commutative for integers, i.e., ...

⇨ Integers are closed for addition & subtraction both. That is, a + b & a – b are again integers, where a & b are any integers. ⇨ Addition is commutative for integers, i.e., ...