Sure thing! We've got 620 and we're gonna multiply it by 21. That's the same as adding 620 to itself 21 times.
Step 1: 0 + 620 = 620
Step 2: 620 + 620 = 1240
Step 3: 1240 + 620 = 1860
Step 4: 1860 + 620 = 2480
Step 5: 2480 + 620 = 3100
Step 6: 3100 + 620 = 3720
Step 7: 3720 + 620 = 4340
Step 8: 4340 + 620 = 4960
Step 9: 4960 + 620 = 5580
Step 10: 5580 + 620 = 6200
Step 11: 6200 + 620 = 6820
Step 12: 6820 + 620 = 7440
Step 13: 7440 + 620 = 8060
Step 14: 8060 + 620 = 8680
Step 15: 8680 + 620 = 9300
Step 16: 9300 + 620 = 9920
Step 17: 9920 + 620 = 10540
Step 18: 10540 + 620 = 11160
Step 19: 11160 + 620 = 11780
Step 20: 11780 + 620 = 12400
Step 21: 12400 + 620 = 13020

So, 620*21 = 13020