Without delay, let's solve this. We've got 660 and we will be multiplying it by 21, that is, adding 660 to itself 21 times.
Step 1: 0 + 660 = 660
Step 2: 660 + 660 = 1320
Step 3: 1320 + 660 = 1980
Step 4: 1980 + 660 = 2640
Step 5: 2640 + 660 = 3300
Step 6: 3300 + 660 = 3960
Step 7: 3960 + 660 = 4620
Step 8: 4620 + 660 = 5280
Step 9: 5280 + 660 = 5940
Step 10: 5940 + 660 = 6600
Step 11: 6600 + 660 = 7260
Step 12: 7260 + 660 = 7920
Step 13: 7920 + 660 = 8580
Step 14: 8580 + 660 = 9240
Step 15: 9240 + 660 = 9900
Step 16: 9900 + 660 = 10560
Step 17: 10560 + 660 = 11220
Step 18: 11220 + 660 = 11880
Step 19: 11880 + 660 = 12540
Step 20: 12540 + 660 = 13200
Step 21: 13200 + 660 = 13860

So, 660*21 = 13860