Sure thing! We've got 644 and we're gonna multiply it by 5. That's the same as adding 644 to itself 5 times.
Step 1: 0 + 644 = 644
Step 2: 644 + 644 = 1288
Step 3: 1288 + 644 = 1932
Step 4: 1932 + 644 = 2576
Step 5: 2576 + 644 = 3220

So, 644*5 = 3220