31•a•20

The expression 31 • a • 20 can be simplified as follows:

31 • a • 20 = (31 • 20) • a

Now we can calculate the bracketed expression:

31 • 20 = 620

So, the simplified expression becomes:

(31 • 20) • a = 620 • a

Therefore, the simplified expression is 620 • a.