Without delay, let's solve this. We've got 665 and we will be multiplying it by 21, that is, adding 665 to itself 21 times.
Step 1: 0 + 665 = 665
Step 2: 665 + 665 = 1330
Step 3: 1330 + 665 = 1995
Step 4: 1995 + 665 = 2660
Step 5: 2660 + 665 = 3325
Step 6: 3325 + 665 = 3990
Step 7: 3990 + 665 = 4655
Step 8: 4655 + 665 = 5320
Step 9: 5320 + 665 = 5985
Step 10: 5985 + 665 = 6650
Step 11: 6650 + 665 = 7315
Step 12: 7315 + 665 = 7980
Step 13: 7980 + 665 = 8645
Step 14: 8645 + 665 = 9310
Step 15: 9310 + 665 = 9975
Step 16: 9975 + 665 = 10640
Step 17: 10640 + 665 = 11305
Step 18: 11305 + 665 = 11970
Step 19: 11970 + 665 = 12635
Step 20: 12635 + 665 = 13300
Step 21: 13300 + 665 = 13965

So, 665*21 = 13965