Let's roll up our sleeves and solve this. We have 121 and we're going to multiply it by 100, essentially adding 121 to itself 100 times.
Step 1: 0 + 121 = 121
Step 2: 121 + 121 = 242
Step 3: 242 + 121 = 363
Step 4: 363 + 121 = 484
Step 5: 484 + 121 = 605
Step 6: 605 + 121 = 726
Step 7: 726 + 121 = 847
Step 8: 847 + 121 = 968
Step 9: 968 + 121 = 1089
Step 10: 1089 + 121 = 1210
Step 11: 1210 + 121 = 1331
Step 12: 1331 + 121 = 1452
Step 13: 1452 + 121 = 1573
Step 14: 1573 + 121 = 1694
Step 15: 1694 + 121 = 1815
Step 16: 1815 + 121 = 1936
Step 17: 1936 + 121 = 2057
Step 18: 2057 + 121 = 2178
Step 19: 2178 + 121 = 2299
Step 20: 2299 + 121 = 2420
Step 21: 2420 + 121 = 2541
Step 22: 2541 + 121 = 2662
Step 23: 2662 + 121 = 2783
Step 24: 2783 + 121 = 2904
Step 25: 2904 + 121 = 3025
Step 26: 3025 + 121 = 3146
Step 27: 3146 + 121 = 3267
Step 28: 3267 + 121 = 3388
Step 29: 3388 + 121 = 3509
Step 30: 3509 + 121 = 3630
Step 31: 3630 + 121 = 3751
Step 32: 3751 + 121 = 3872
Step 33: 3872 + 121 = 3993
Step 34: 3993 + 121 = 4114
Step 35: 4114 + 121 = 4235
Step 36: 4235 + 121 = 4356
Step 37: 4356 + 121 = 4477
Step 38: 4477 + 121 = 4598
Step 39: 4598 + 121 = 4719
Step 40: 4719 + 121 = 4840
Step 41: 4840 + 121 = 4961
Step 42: 4961 + 121 = 5082
Step 43: 5082 + 121 = 5203
Step 44: 5203 + 121 = 5324
Step 45: 5324 + 121 = 5445
Step 46: 5445 + 121 = 5566
Step 47: 5566 + 121 = 5687
Step 48: 5687 + 121 = 5808
Step 49: 5808 + 121 = 5929
Step 50: 5929 + 121 = 6050
Step 51: 6050 + 121 = 6171
Step 52: 6171 + 121 = 6292
Step 53: 6292 + 121 = 6413
Step 54: 6413 + 121 = 6534
Step 55: 6534 + 121 = 6655
Step 56: 6655 + 121 = 6776
Step 57: 6776 + 121 = 6897
Step 58: 6897 + 121 = 7018
Step 59: 7018 + 121 = 7139
Step 60: 7139 + 121 = 7260
Step 61: 7260 + 121 = 7381
Step 62: 7381 + 121 = 7502
Step 63: 7502 + 121 = 7623
Step 64: 7623 + 121 = 7744
Step 65: 7744 + 121 = 7865
Step 66: 7865 + 121 = 7986
Step 67: 7986 + 121 = 8107
Step 68: 8107 + 121 = 8228
Step 69: 8228 + 121 = 8349
Step 70: 8349 + 121 = 8470
Step 71: 8470 + 121 = 8591
Step 72: 8591 + 121 = 8712
Step 73: 8712 + 121 = 8833
Step 74: 8833 + 121 = 8954
Step 75: 8954 + 121 = 9075
Step 76: 9075 + 121 = 9196
Step 77: 9196 + 121 = 9317
Step 78: 9317 + 121 = 9438
Step 79: 9438 + 121 = 9559
Step 80: 9559 + 121 = 9680
Step 81: 9680 + 121 = 9801
Step 82: 9801 + 121 = 9922
Step 83: 9922 + 121 = 10043
Step 84: 10043 + 121 = 10164
Step 85: 10164 + 121 = 10285
Step 86: 10285 + 121 = 10406
Step 87: 10406 + 121 = 10527
Step 88: 10527 + 121 = 10648
Step 89: 10648 + 121 = 10769
Step 90: 10769 + 121 = 10890
Step 91: 10890 + 121 = 11011
Step 92: 11011 + 121 = 11132
Step 93: 11132 + 121 = 11253
Step 94: 11253 + 121 = 11374
Step 95: 11374 + 121 = 11495
Step 96: 11495 + 121 = 11616
Step 97: 11616 + 121 = 11737
Step 98: 11737 + 121 = 11858
Step 99: 11858 + 121 = 11979
Step 100: 11979 + 121 = 12100

So, 121*100 = 12100