Let's roll up our sleeves and solve this. We have 421 and we're going to multiply it by 423, essentially adding 421 to itself 423 times.
Step 1: 0 + 421 = 421
Step 2: 421 + 421 = 842
Step 3: 842 + 421 = 1263
Step 4: 1263 + 421 = 1684
Step 5: 1684 + 421 = 2105
Step 6: 2105 + 421 = 2526
Step 7: 2526 + 421 = 2947
Step 8: 2947 + 421 = 3368
Step 9: 3368 + 421 = 3789
Step 10: 3789 + 421 = 4210
Step 11: 4210 + 421 = 4631
Step 12: 4631 + 421 = 5052
Step 13: 5052 + 421 = 5473
Step 14: 5473 + 421 = 5894
Step 15: 5894 + 421 = 6315
Step 16: 6315 + 421 = 6736
Step 17: 6736 + 421 = 7157
Step 18: 7157 + 421 = 7578
Step 19: 7578 + 421 = 7999
Step 20: 7999 + 421 = 8420
Step 21: 8420 + 421 = 8841
Step 22: 8841 + 421 = 9262
Step 23: 9262 + 421 = 9683
Step 24: 9683 + 421 = 10104
Step 25: 10104 + 421 = 10525
Step 26: 10525 + 421 = 10946
Step 27: 10946 + 421 = 11367
Step 28: 11367 + 421 = 11788
Step 29: 11788 + 421 = 12209
Step 30: 12209 + 421 = 12630
Step 31: 12630 + 421 = 13051
Step 32: 13051 + 421 = 13472
Step 33: 13472 + 421 = 13893
Step 34: 13893 + 421 = 14314
Step 35: 14314 + 421 = 14735
Step 36: 14735 + 421 = 15156
Step 37: 15156 + 421 = 15577
Step 38: 15577 + 421 = 15998
Step 39: 15998 + 421 = 16419
Step 40: 16419 + 421 = 16840
Step 41: 16840 + 421 = 17261
Step 42: 17261 + 421 = 17682
Step 43: 17682 + 421 = 18103
Step 44: 18103 + 421 = 18524
Step 45: 18524 + 421 = 18945
Step 46: 18945 + 421 = 19366
Step 47: 19366 + 421 = 19787
Step 48: 19787 + 421 = 20208
Step 49: 20208 + 421 = 20629
Step 50: 20629 + 421 = 21050
Step 51: 21050 + 421 = 21471
Step 52: 21471 + 421 = 21892
Step 53: 21892 + 421 = 22313
Step 54: 22313 + 421 = 22734
Step 55: 22734 + 421 = 23155
Step 56: 23155 + 421 = 23576
Step 57: 23576 + 421 = 23997
Step 58: 23997 + 421 = 24418
Step 59: 24418 + 421 = 24839
Step 60: 24839 + 421 = 25260
Step 61: 25260 + 421 = 25681
Step 62: 25681 + 421 = 26102
Step 63: 26102 + 421 = 26523
Step 64: 26523 + 421 = 26944
Step 65: 26944 + 421 = 27365
Step 66: 27365 + 421 = 27786
Step 67: 27786 + 421 = 28207
Step 68: 28207 + 421 = 28628
Step 69: 28628 + 421 = 29049
Step 70: 29049 + 421 = 29470
Step 71: 29470 + 421 = 29891
Step 72: 29891 + 421 = 30312
Step 73: 30312 + 421 = 30733
Step 74: 30733 + 421 = 31154
Step 75: 31154 + 421 = 31575
Step 76: 31575 + 421 = 31996
Step 77: 31996 + 421 = 32417
Step 78: 32417 + 421 = 32838
Step 79: 32838 + 421 = 33259
Step 80: 33259 + 421 = 33680
Step 81: 33680 + 421 = 34101
Step 82: 34101 + 421 = 34522
Step 83: 34522 + 421 = 34943
Step 84: 34943 + 421 = 35364
Step 85: 35364 + 421 = 35785
Step 86: 35785 + 421 = 36206
Step 87: 36206 + 421 = 36627
Step 88: 36627 + 421 = 37048
Step 89: 37048 + 421 = 37469
Step 90: 37469 + 421 = 37890
Step 91: 37890 + 421 = 38311
Step 92: 38311 + 421 = 38732
Step 93: 38732 + 421 = 39153
Step 94: 39153 + 421 = 39574
Step 95: 39574 + 421 = 39995
Step 96: 39995 + 421 = 40416
Step 97: 40416 + 421 = 40837
Step 98: 40837 + 421 = 41258
Step 99: 41258 + 421 = 41679
Step 100: 41679 + 421 = 42100
Step 101: 42100 + 421 = 42521
Step 102: 42521 + 421 = 42942
Step 103: 42942 + 421 = 43363
Step 104: 43363 + 421 = 43784
Step 105: 43784 + 421 = 44205
Step 106: 44205 + 421 = 44626
Step 107: 44626 + 421 = 45047
Step 108: 45047 + 421 = 45468
Step 109: 45468 + 421 = 45889
Step 110: 45889 + 421 = 46310
Step 111: 46310 + 421 = 46731
Step 112: 46731 + 421 = 47152
Step 113: 47152 + 421 = 47573
Step 114: 47573 + 421 = 47994
Step 115: 47994 + 421 = 48415
Step 116: 48415 + 421 = 48836
Step 117: 48836 + 421 = 49257
Step 118: 49257 + 421 = 49678
Step 119: 49678 + 421 = 50099
Step 120: 50099 + 421 = 50520
Step 121: 50520 + 421 = 50941
Step 122: 50941 + 421 = 51362
Step 123: 51362 + 421 = 51783
Step 124: 51783 + 421 = 52204
Step 125: 52204 + 421 = 52625
Step 126: 52625 + 421 = 53046
Step 127: 53046 + 421 = 53467
Step 128: 53467 + 421 = 53888
Step 129: 53888 + 421 = 54309
Step 130: 54309 + 421 = 54730
Step 131: 54730 + 421 = 55151
Step 132: 55151 + 421 = 55572
Step 133: 55572 + 421 = 55993
Step 134: 55993 + 421 = 56414
Step 135: 56414 + 421 = 56835
Step 136: 56835 + 421 = 57256
Step 137: 57256 + 421 = 57677
Step 138: 57677 + 421 = 58098
Step 139: 58098 + 421 = 58519
Step 140: 58519 + 421 = 58940
Step 141: 58940 + 421 = 59361
Step 142: 59361 + 421 = 59782
Step 143: 59782 + 421 = 60203
Step 144: 60203 + 421 = 60624
Step 145: 60624 + 421 = 61045
Step 146: 61045 + 421 = 61466
Step 147: 61466 + 421 = 61887
Step 148: 61887 + 421 = 62308
Step 149: 62308 + 421 = 62729
Step 150: 62729 + 421 = 63150
Step 151: 63150 + 421 = 63571
Step 152: 63571 + 421 = 63992
Step 153: 63992 + 421 = 64413
Step 154: 64413 + 421 = 64834
Step 155: 64834 + 421 = 65255
Step 156: 65255 + 421 = 65676
Step 157: 65676 + 421 = 66097
Step 158: 66097 + 421 = 66518
Step 159: 66518 + 421 = 66939
Step 160: 66939 + 421 = 67360
Step 161: 67360 + 421 = 67781
Step 162: 67781 + 421 = 68202
Step 163: 68202 + 421 = 68623
Step 164: 68623 + 421 = 69044
Step 165: 69044 + 421 = 69465
Step 166: 69465 + 421 = 69886
Step 167: 69886 + 421 = 70307
Step 168: 70307 + 421 = 70728
Step 169: 70728 + 421 = 71149
Step 170: 71149 + 421 = 71570
Step 171: 71570 + 421 = 71991
Step 172: 71991 + 421 = 72412
Step 173: 72412 + 421 = 72833
Step 174: 72833 + 421 = 73254
Step 175: 73254 + 421 = 73675
Step 176: 73675 + 421 = 74096
Step 177: 74096 + 421 = 74517
Step 178: 74517 + 421 = 74938
Step 179: 74938 + 421 = 75359
Step 180: 75359 + 421 = 75780
Step 181: 75780 + 421 = 76201
Step 182: 76201 + 421 = 76622
Step 183: 76622 + 421 = 77043
Step 184: 77043 + 421 = 77464
Step 185: 77464 + 421 = 77885
Step 186: 77885 + 421 = 78306
Step 187: 78306 + 421 = 78727
Step 188: 78727 + 421 = 79148
Step 189: 79148 + 421 = 79569
Step 190: 79569 + 421 = 79990
Step 191: 79990 + 421 = 80411
Step 192: 80411 + 421 = 80832
Step 193: 80832 + 421 = 81253
Step 194: 81253 + 421 = 81674
Step 195: 81674 + 421 = 82095
Step 196: 82095 + 421 = 82516
Step 197: 82516 + 421 = 82937
Step 198: 82937 + 421 = 83358
Step 199: 83358 + 421 = 83779
Step 200: 83779 + 421 = 84200
Step 201: 84200 + 421 = 84621
Step 202: 84621 + 421 = 85042
Step 203: 85042 + 421 = 85463
Step 204: 85463 + 421 = 85884
Step 205: 85884 + 421 = 86305
Step 206: 86305 + 421 = 86726
Step 207: 86726 + 421 = 87147
Step 208: 87147 + 421 = 87568
Step 209: 87568 + 421 = 87989
Step 210: 87989 + 421 = 88410
Step 211: 88410 + 421 = 88831
Step 212: 88831 + 421 = 89252
Step 213: 89252 + 421 = 89673
Step 214: 89673 + 421 = 90094
Step 215: 90094 + 421 = 90515
Step 216: 90515 + 421 = 90936
Step 217: 90936 + 421 = 91357
Step 218: 91357 + 421 = 91778
Step 219: 91778 + 421 = 92199
Step 220: 92199 + 421 = 92620
Step 221: 92620 + 421 = 93041
Step 222: 93041 + 421 = 93462
Step 223: 93462 + 421 = 93883
Step 224: 93883 + 421 = 94304
Step 225: 94304 + 421 = 94725
Step 226: 94725 + 421 = 95146
Step 227: 95146 + 421 = 95567
Step 228: 95567 + 421 = 95988
Step 229: 95988 + 421 = 96409
Step 230: 96409 + 421 = 96830
Step 231: 96830 + 421 = 97251
Step 232: 97251 + 421 = 97672
Step 233: 97672 + 421 = 98093
Step 234: 98093 + 421 = 98514
Step 235: 98514 + 421 = 98935
Step 236: 98935 + 421 = 99356
Step 237: 99356 + 421 = 99777
Step 238: 99777 + 421 = 100198
Step 239: 100198 + 421 = 100619
Step 240: 100619 + 421 = 101040
Step 241: 101040 + 421 = 101461
Step 242: 101461 + 421 = 101882
Step 243: 101882 + 421 = 102303
Step 244: 102303 + 421 = 102724
Step 245: 102724 + 421 = 103145
Step 246: 103145 + 421 = 103566
Step 247: 103566 + 421 = 103987
Step 248: 103987 + 421 = 104408
Step 249: 104408 + 421 = 104829
Step 250: 104829 + 421 = 105250
Step 251: 105250 + 421 = 105671
Step 252: 105671 + 421 = 106092
Step 253: 106092 + 421 = 106513
Step 254: 106513 + 421 = 106934
Step 255: 106934 + 421 = 107355
Step 256: 107355 + 421 = 107776
Step 257: 107776 + 421 = 108197
Step 258: 108197 + 421 = 108618
Step 259: 108618 + 421 = 109039
Step 260: 109039 + 421 = 109460
Step 261: 109460 + 421 = 109881
Step 262: 109881 + 421 = 110302
Step 263: 110302 + 421 = 110723
Step 264: 110723 + 421 = 111144
Step 265: 111144 + 421 = 111565
Step 266: 111565 + 421 = 111986
Step 267: 111986 + 421 = 112407
Step 268: 112407 + 421 = 112828
Step 269: 112828 + 421 = 113249
Step 270: 113249 + 421 = 113670
Step 271: 113670 + 421 = 114091
Step 272: 114091 + 421 = 114512
Step 273: 114512 + 421 = 114933
Step 274: 114933 + 421 = 115354
Step 275: 115354 + 421 = 115775
Step 276: 115775 + 421 = 116196
Step 277: 116196 + 421 = 116617
Step 278: 116617 + 421 = 117038
Step 279: 117038 + 421 = 117459
Step 280: 117459 + 421 = 117880
Step 281: 117880 + 421 = 118301
Step 282: 118301 + 421 = 118722
Step 283: 118722 + 421 = 119143
Step 284: 119143 + 421 = 119564
Step 285: 119564 + 421 = 119985
Step 286: 119985 + 421 = 120406
Step 287: 120406 + 421 = 120827
Step 288: 120827 + 421 = 121248
Step 289: 121248 + 421 = 121669
Step 290: 121669 + 421 = 122090
Step 291: 122090 + 421 = 122511
Step 292: 122511 + 421 = 122932
Step 293: 122932 + 421 = 123353
Step 294: 123353 + 421 = 123774
Step 295: 123774 + 421 = 124195
Step 296: 124195 + 421 = 124616
Step 297: 124616 + 421 = 125037
Step 298: 125037 + 421 = 125458
Step 299: 125458 + 421 = 125879
Step 300: 125879 + 421 = 126300
Step 301: 126300 + 421 = 126721
Step 302: 126721 + 421 = 127142
Step 303: 127142 + 421 = 127563
Step 304: 127563 + 421 = 127984
Step 305: 127984 + 421 = 128405
Step 306: 128405 + 421 = 128826
Step 307: 128826 + 421 = 129247
Step 308: 129247 + 421 = 129668
Step 309: 129668 + 421 = 130089
Step 310: 130089 + 421 = 130510
Step 311: 130510 + 421 = 130931
Step 312: 130931 + 421 = 131352
Step 313: 131352 + 421 = 131773
Step 314: 131773 + 421 = 132194
Step 315: 132194 + 421 = 132615
Step 316: 132615 + 421 = 133036
Step 317: 133036 + 421 = 133457
Step 318: 133457 + 421 = 133878
Step 319: 133878 + 421 = 134299
Step 320: 134299 + 421 = 134720
Step 321: 134720 + 421 = 135141
Step 322: 135141 + 421 = 135562
Step 323: 135562 + 421 = 135983
Step 324: 135983 + 421 = 136404
Step 325: 136404 + 421 = 136825
Step 326: 136825 + 421 = 137246
Step 327: 137246 + 421 = 137667
Step 328: 137667 + 421 = 138088
Step 329: 138088 + 421 = 138509
Step 330: 138509 + 421 = 138930
Step 331: 138930 + 421 = 139351
Step 332: 139351 + 421 = 139772
Step 333: 139772 + 421 = 140193
Step 334: 140193 + 421 = 140614
Step 335: 140614 + 421 = 141035
Step 336: 141035 + 421 = 141456
Step 337: 141456 + 421 = 141877
Step 338: 141877 + 421 = 142298
Step 339: 142298 + 421 = 142719
Step 340: 142719 + 421 = 143140
Step 341: 143140 + 421 = 143561
Step 342: 143561 + 421 = 143982
Step 343: 143982 + 421 = 144403
Step 344: 144403 + 421 = 144824
Step 345: 144824 + 421 = 145245
Step 346: 145245 + 421 = 145666
Step 347: 145666 + 421 = 146087
Step 348: 146087 + 421 = 146508
Step 349: 146508 + 421 = 146929
Step 350: 146929 + 421 = 147350
Step 351: 147350 + 421 = 147771
Step 352: 147771 + 421 = 148192
Step 353: 148192 + 421 = 148613
Step 354: 148613 + 421 = 149034
Step 355: 149034 + 421 = 149455
Step 356: 149455 + 421 = 149876
Step 357: 149876 + 421 = 150297
Step 358: 150297 + 421 = 150718
Step 359: 150718 + 421 = 151139
Step 360: 151139 + 421 = 151560
Step 361: 151560 + 421 = 151981
Step 362: 151981 + 421 = 152402
Step 363: 152402 + 421 = 152823
Step 364: 152823 + 421 = 153244
Step 365: 153244 + 421 = 153665
Step 366: 153665 + 421 = 154086
Step 367: 154086 + 421 = 154507
Step 368: 154507 + 421 = 154928
Step 369: 154928 + 421 = 155349
Step 370: 155349 + 421 = 155770
Step 371: 155770 + 421 = 156191
Step 372: 156191 + 421 = 156612
Step 373: 156612 + 421 = 157033
Step 374: 157033 + 421 = 157454
Step 375: 157454 + 421 = 157875
Step 376: 157875 + 421 = 158296
Step 377: 158296 + 421 = 158717
Step 378: 158717 + 421 = 159138
Step 379: 159138 + 421 = 159559
Step 380: 159559 + 421 = 159980
Step 381: 159980 + 421 = 160401
Step 382: 160401 + 421 = 160822
Step 383: 160822 + 421 = 161243
Step 384: 161243 + 421 = 161664
Step 385: 161664 + 421 = 162085
Step 386: 162085 + 421 = 162506
Step 387: 162506 + 421 = 162927
Step 388: 162927 + 421 = 163348
Step 389: 163348 + 421 = 163769
Step 390: 163769 + 421 = 164190
Step 391: 164190 + 421 = 164611
Step 392: 164611 + 421 = 165032
Step 393: 165032 + 421 = 165453
Step 394: 165453 + 421 = 165874
Step 395: 165874 + 421 = 166295
Step 396: 166295 + 421 = 166716
Step 397: 166716 + 421 = 167137
Step 398: 167137 + 421 = 167558
Step 399: 167558 + 421 = 167979
Step 400: 167979 + 421 = 168400
Step 401: 168400 + 421 = 168821
Step 402: 168821 + 421 = 169242
Step 403: 169242 + 421 = 169663
Step 404: 169663 + 421 = 170084
Step 405: 170084 + 421 = 170505
Step 406: 170505 + 421 = 170926
Step 407: 170926 + 421 = 171347
Step 408: 171347 + 421 = 171768
Step 409: 171768 + 421 = 172189
Step 410: 172189 + 421 = 172610
Step 411: 172610 + 421 = 173031
Step 412: 173031 + 421 = 173452
Step 413: 173452 + 421 = 173873
Step 414: 173873 + 421 = 174294
Step 415: 174294 + 421 = 174715
Step 416: 174715 + 421 = 175136
Step 417: 175136 + 421 = 175557
Step 418: 175557 + 421 = 175978
Step 419: 175978 + 421 = 176399
Step 420: 176399 + 421 = 176820
Step 421: 176820 + 421 = 177241
Step 422: 177241 + 421 = 177662
Step 423: 177662 + 421 = 178083

So, 421*423 = 178083