Let's get this math done. We have 645 and we're going to multiply it by 4. This is the same as taking 645 and adding it to itself 4 times.
Step 1: 0 + 645 = 645
Step 2: 645 + 645 = 1290
Step 3: 1290 + 645 = 1935
Step 4: 1935 + 645 = 2580

So, 645*4 = 2580