For example, evaluate 17*5*20:
Method 1: (17*5)*20 = 85*20 = 1,700.
Method 2: 17*(5*20) = 17*100 = 1,700.
Notice that by first multiplying 5*20 to get 100, the calculation
is easy to do in your head.
You can also use the commutative property to rearrange a question:
5*21*4 = 5*4*21 = 20*21 = 420