Let's get this math done. We have 643 and we're going to multiply it by 5. This is the same as taking 643 and adding it to itself 5 times.
Step 1: 0 + 643 = 643
Step 2: 643 + 643 = 1286
Step 3: 1286 + 643 = 1929
Step 4: 1929 + 643 = 2572
Step 5: 2572 + 643 = 3215

So, 643*5 = 3215