Without delay, let's solve this. We've got 853 and we will be multiplying it by 21, that is, adding 853 to itself 21 times.
Step 1: 0 + 853 = 853
Step 2: 853 + 853 = 1706
Step 3: 1706 + 853 = 2559
Step 4: 2559 + 853 = 3412
Step 5: 3412 + 853 = 4265
Step 6: 4265 + 853 = 5118
Step 7: 5118 + 853 = 5971
Step 8: 5971 + 853 = 6824
Step 9: 6824 + 853 = 7677
Step 10: 7677 + 853 = 8530
Step 11: 8530 + 853 = 9383
Step 12: 9383 + 853 = 10236
Step 13: 10236 + 853 = 11089
Step 14: 11089 + 853 = 11942
Step 15: 11942 + 853 = 12795
Step 16: 12795 + 853 = 13648
Step 17: 13648 + 853 = 14501
Step 18: 14501 + 853 = 15354
Step 19: 15354 + 853 = 16207
Step 20: 16207 + 853 = 17060
Step 21: 17060 + 853 = 17913

So, 853*21 = 17913