Algebra Rule 9
Multiplication and division (by the same number) cancel each other out
```{ac+bc \over c} = a+b```
Description:
Division is the inverse of multiplication: if ``{a \over b} = c`` then ``b*c = a``. This means that the fraction ``{ac \over c}`` is equal to ``a``, since we are multiplying ``a`` by ``c`` and then immediately dividing it by ``c`` again, which puts us right back where we started. Since we know that ``{ac+bc \over c} = {ac \over c} + {bc \over c}`` (see rule 8), and based on the above we can see that ``{ac \over c} = a`` and ``{bc \over c} = b``, we have our result: ``a+b``.
```{(4*5)+(2*5) \over 5} = {4*5 \over 5}+{2*5 \over 5} = 4+2```