OK, let's crack this. We're given 646 and our task is to multiply it by 20. Essentially, we'll be adding 646 to itself 20 times.
Step 1: 0 + 646 = 646
Step 2: 646 + 646 = 1292
Step 3: 1292 + 646 = 1938
Step 4: 1938 + 646 = 2584
Step 5: 2584 + 646 = 3230
Step 6: 3230 + 646 = 3876
Step 7: 3876 + 646 = 4522
Step 8: 4522 + 646 = 5168
Step 9: 5168 + 646 = 5814
Step 10: 5814 + 646 = 6460
Step 11: 6460 + 646 = 7106
Step 12: 7106 + 646 = 7752
Step 13: 7752 + 646 = 8398
Step 14: 8398 + 646 = 9044
Step 15: 9044 + 646 = 9690
Step 16: 9690 + 646 = 10336
Step 17: 10336 + 646 = 10982
Step 18: 10982 + 646 = 11628
Step 19: 11628 + 646 = 12274
Step 20: 12274 + 646 = 12920

So, 646*20 = 12920