Sure thing, let's get straight to it. We start with 821 and we're going to multiply it by 21, which means adding 821 to itself 21 times.
Step 1: 0 + 821 = 821
Step 2: 821 + 821 = 1642
Step 3: 1642 + 821 = 2463
Step 4: 2463 + 821 = 3284
Step 5: 3284 + 821 = 4105
Step 6: 4105 + 821 = 4926
Step 7: 4926 + 821 = 5747
Step 8: 5747 + 821 = 6568
Step 9: 6568 + 821 = 7389
Step 10: 7389 + 821 = 8210
Step 11: 8210 + 821 = 9031
Step 12: 9031 + 821 = 9852
Step 13: 9852 + 821 = 10673
Step 14: 10673 + 821 = 11494
Step 15: 11494 + 821 = 12315
Step 16: 12315 + 821 = 13136
Step 17: 13136 + 821 = 13957
Step 18: 13957 + 821 = 14778
Step 19: 14778 + 821 = 15599
Step 20: 15599 + 821 = 16420
Step 21: 16420 + 821 = 17241

So, 821*21 = 17241