Sometimes in mental arithmetic, calculations are done in a different order because the numbers are more "convenient." For example, consider the following calculations:

$\displaystyle{13}\cdot{2}\cdot{5}={26}\cdot{5}={130}\usepackage{cmbright}\Huge\$

and

$\displaystyle{13}\cdot{2}\cdot{5}={13}\cdot{10}={130}\usepackage{cmbright}\Huge\$

The first calculation was performed left-to-right, according to the standard rule for several multiplication calculations. However, the second calculation is much simpler because it was done in a different order.

Which of the following mathematical properties allows us to do this?