OK, let's work through this together. We're starting with 637 and we're going to multiply it by 81, which means we add 637 to itself 81 times.
Step 1: 0 + 637 = 637
Step 2: 637 + 637 = 1274
Step 3: 1274 + 637 = 1911
Step 4: 1911 + 637 = 2548
Step 5: 2548 + 637 = 3185
Step 6: 3185 + 637 = 3822
Step 7: 3822 + 637 = 4459
Step 8: 4459 + 637 = 5096
Step 9: 5096 + 637 = 5733
Step 10: 5733 + 637 = 6370
Step 11: 6370 + 637 = 7007
Step 12: 7007 + 637 = 7644
Step 13: 7644 + 637 = 8281
Step 14: 8281 + 637 = 8918
Step 15: 8918 + 637 = 9555
Step 16: 9555 + 637 = 10192
Step 17: 10192 + 637 = 10829
Step 18: 10829 + 637 = 11466
Step 19: 11466 + 637 = 12103
Step 20: 12103 + 637 = 12740
Step 21: 12740 + 637 = 13377
Step 22: 13377 + 637 = 14014
Step 23: 14014 + 637 = 14651
Step 24: 14651 + 637 = 15288
Step 25: 15288 + 637 = 15925
Step 26: 15925 + 637 = 16562
Step 27: 16562 + 637 = 17199
Step 28: 17199 + 637 = 17836
Step 29: 17836 + 637 = 18473
Step 30: 18473 + 637 = 19110
Step 31: 19110 + 637 = 19747
Step 32: 19747 + 637 = 20384
Step 33: 20384 + 637 = 21021
Step 34: 21021 + 637 = 21658
Step 35: 21658 + 637 = 22295
Step 36: 22295 + 637 = 22932
Step 37: 22932 + 637 = 23569
Step 38: 23569 + 637 = 24206
Step 39: 24206 + 637 = 24843
Step 40: 24843 + 637 = 25480
Step 41: 25480 + 637 = 26117
Step 42: 26117 + 637 = 26754
Step 43: 26754 + 637 = 27391
Step 44: 27391 + 637 = 28028
Step 45: 28028 + 637 = 28665
Step 46: 28665 + 637 = 29302
Step 47: 29302 + 637 = 29939
Step 48: 29939 + 637 = 30576
Step 49: 30576 + 637 = 31213
Step 50: 31213 + 637 = 31850
Step 51: 31850 + 637 = 32487
Step 52: 32487 + 637 = 33124
Step 53: 33124 + 637 = 33761
Step 54: 33761 + 637 = 34398
Step 55: 34398 + 637 = 35035
Step 56: 35035 + 637 = 35672
Step 57: 35672 + 637 = 36309
Step 58: 36309 + 637 = 36946
Step 59: 36946 + 637 = 37583
Step 60: 37583 + 637 = 38220
Step 61: 38220 + 637 = 38857
Step 62: 38857 + 637 = 39494
Step 63: 39494 + 637 = 40131
Step 64: 40131 + 637 = 40768
Step 65: 40768 + 637 = 41405
Step 66: 41405 + 637 = 42042
Step 67: 42042 + 637 = 42679
Step 68: 42679 + 637 = 43316
Step 69: 43316 + 637 = 43953
Step 70: 43953 + 637 = 44590
Step 71: 44590 + 637 = 45227
Step 72: 45227 + 637 = 45864
Step 73: 45864 + 637 = 46501
Step 74: 46501 + 637 = 47138
Step 75: 47138 + 637 = 47775
Step 76: 47775 + 637 = 48412
Step 77: 48412 + 637 = 49049
Step 78: 49049 + 637 = 49686
Step 79: 49686 + 637 = 50323
Step 80: 50323 + 637 = 50960
Step 81: 50960 + 637 = 51597

So, 637*81 = 51597