a. A domino set contains all number pairs from double-0 to double-6, with each number pair...

a. A domino set contains all number pairs from double-0 to double-6, with each number pair occurring only once; for example, the following domino counts as 2-4 and 4-2. How many dominoes are in the set?

b. When considering the sum of all dots on a single domino in an ordinary set of dominoes, explain how the commutative property might be important.