OK, let's work through this together. We're starting with 641 and we're going to multiply it by 13, which means we add 641 to itself 13 times.
Step 1: 0 + 641 = 641
Step 2: 641 + 641 = 1282
Step 3: 1282 + 641 = 1923
Step 4: 1923 + 641 = 2564
Step 5: 2564 + 641 = 3205
Step 6: 3205 + 641 = 3846
Step 7: 3846 + 641 = 4487
Step 8: 4487 + 641 = 5128
Step 9: 5128 + 641 = 5769
Step 10: 5769 + 641 = 6410
Step 11: 6410 + 641 = 7051
Step 12: 7051 + 641 = 7692
Step 13: 7692 + 641 = 8333

So, 641*13 = 8333