The property of 16 + 31 = 31 + 16 is called Commutative Property.
The property states that the order does not matter. This includes both multiplication and addition. Other examples include 4 + 2 = 2 + 4, or even 10 + 9 + 3 = 3 + 9 + 10. In easier words, the problem is just the backwards version of the original problem.
