Sure thing! We've got 855 and we're gonna multiply it by 21. That's the same as adding 855 to itself 21 times.
Step 1: 0 + 855 = 855
Step 2: 855 + 855 = 1710
Step 3: 1710 + 855 = 2565
Step 4: 2565 + 855 = 3420
Step 5: 3420 + 855 = 4275
Step 6: 4275 + 855 = 5130
Step 7: 5130 + 855 = 5985
Step 8: 5985 + 855 = 6840
Step 9: 6840 + 855 = 7695
Step 10: 7695 + 855 = 8550
Step 11: 8550 + 855 = 9405
Step 12: 9405 + 855 = 10260
Step 13: 10260 + 855 = 11115
Step 14: 11115 + 855 = 11970
Step 15: 11970 + 855 = 12825
Step 16: 12825 + 855 = 13680
Step 17: 13680 + 855 = 14535
Step 18: 14535 + 855 = 15390
Step 19: 15390 + 855 = 16245
Step 20: 16245 + 855 = 17100
Step 21: 17100 + 855 = 17955

So, 855*21 = 17955