Let's roll up our sleeves and solve this. We have 641 and we're going to multiply it by 158, essentially adding 641 to itself 158 times.
Step 1: 0 + 641 = 641
Step 2: 641 + 641 = 1282
Step 3: 1282 + 641 = 1923
Step 4: 1923 + 641 = 2564
Step 5: 2564 + 641 = 3205
Step 6: 3205 + 641 = 3846
Step 7: 3846 + 641 = 4487
Step 8: 4487 + 641 = 5128
Step 9: 5128 + 641 = 5769
Step 10: 5769 + 641 = 6410
Step 11: 6410 + 641 = 7051
Step 12: 7051 + 641 = 7692
Step 13: 7692 + 641 = 8333
Step 14: 8333 + 641 = 8974
Step 15: 8974 + 641 = 9615
Step 16: 9615 + 641 = 10256
Step 17: 10256 + 641 = 10897
Step 18: 10897 + 641 = 11538
Step 19: 11538 + 641 = 12179
Step 20: 12179 + 641 = 12820
Step 21: 12820 + 641 = 13461
Step 22: 13461 + 641 = 14102
Step 23: 14102 + 641 = 14743
Step 24: 14743 + 641 = 15384
Step 25: 15384 + 641 = 16025
Step 26: 16025 + 641 = 16666
Step 27: 16666 + 641 = 17307
Step 28: 17307 + 641 = 17948
Step 29: 17948 + 641 = 18589
Step 30: 18589 + 641 = 19230
Step 31: 19230 + 641 = 19871
Step 32: 19871 + 641 = 20512
Step 33: 20512 + 641 = 21153
Step 34: 21153 + 641 = 21794
Step 35: 21794 + 641 = 22435
Step 36: 22435 + 641 = 23076
Step 37: 23076 + 641 = 23717
Step 38: 23717 + 641 = 24358
Step 39: 24358 + 641 = 24999
Step 40: 24999 + 641 = 25640
Step 41: 25640 + 641 = 26281
Step 42: 26281 + 641 = 26922
Step 43: 26922 + 641 = 27563
Step 44: 27563 + 641 = 28204
Step 45: 28204 + 641 = 28845
Step 46: 28845 + 641 = 29486
Step 47: 29486 + 641 = 30127
Step 48: 30127 + 641 = 30768
Step 49: 30768 + 641 = 31409
Step 50: 31409 + 641 = 32050
Step 51: 32050 + 641 = 32691
Step 52: 32691 + 641 = 33332
Step 53: 33332 + 641 = 33973
Step 54: 33973 + 641 = 34614
Step 55: 34614 + 641 = 35255
Step 56: 35255 + 641 = 35896
Step 57: 35896 + 641 = 36537
Step 58: 36537 + 641 = 37178
Step 59: 37178 + 641 = 37819
Step 60: 37819 + 641 = 38460
Step 61: 38460 + 641 = 39101
Step 62: 39101 + 641 = 39742
Step 63: 39742 + 641 = 40383
Step 64: 40383 + 641 = 41024
Step 65: 41024 + 641 = 41665
Step 66: 41665 + 641 = 42306
Step 67: 42306 + 641 = 42947
Step 68: 42947 + 641 = 43588
Step 69: 43588 + 641 = 44229
Step 70: 44229 + 641 = 44870
Step 71: 44870 + 641 = 45511
Step 72: 45511 + 641 = 46152
Step 73: 46152 + 641 = 46793
Step 74: 46793 + 641 = 47434
Step 75: 47434 + 641 = 48075
Step 76: 48075 + 641 = 48716
Step 77: 48716 + 641 = 49357
Step 78: 49357 + 641 = 49998
Step 79: 49998 + 641 = 50639
Step 80: 50639 + 641 = 51280
Step 81: 51280 + 641 = 51921
Step 82: 51921 + 641 = 52562
Step 83: 52562 + 641 = 53203
Step 84: 53203 + 641 = 53844
Step 85: 53844 + 641 = 54485
Step 86: 54485 + 641 = 55126
Step 87: 55126 + 641 = 55767
Step 88: 55767 + 641 = 56408
Step 89: 56408 + 641 = 57049
Step 90: 57049 + 641 = 57690
Step 91: 57690 + 641 = 58331
Step 92: 58331 + 641 = 58972
Step 93: 58972 + 641 = 59613
Step 94: 59613 + 641 = 60254
Step 95: 60254 + 641 = 60895
Step 96: 60895 + 641 = 61536
Step 97: 61536 + 641 = 62177
Step 98: 62177 + 641 = 62818
Step 99: 62818 + 641 = 63459
Step 100: 63459 + 641 = 64100
Step 101: 64100 + 641 = 64741
Step 102: 64741 + 641 = 65382
Step 103: 65382 + 641 = 66023
Step 104: 66023 + 641 = 66664
Step 105: 66664 + 641 = 67305
Step 106: 67305 + 641 = 67946
Step 107: 67946 + 641 = 68587
Step 108: 68587 + 641 = 69228
Step 109: 69228 + 641 = 69869
Step 110: 69869 + 641 = 70510
Step 111: 70510 + 641 = 71151
Step 112: 71151 + 641 = 71792
Step 113: 71792 + 641 = 72433
Step 114: 72433 + 641 = 73074
Step 115: 73074 + 641 = 73715
Step 116: 73715 + 641 = 74356
Step 117: 74356 + 641 = 74997
Step 118: 74997 + 641 = 75638
Step 119: 75638 + 641 = 76279
Step 120: 76279 + 641 = 76920
Step 121: 76920 + 641 = 77561
Step 122: 77561 + 641 = 78202
Step 123: 78202 + 641 = 78843
Step 124: 78843 + 641 = 79484
Step 125: 79484 + 641 = 80125
Step 126: 80125 + 641 = 80766
Step 127: 80766 + 641 = 81407
Step 128: 81407 + 641 = 82048
Step 129: 82048 + 641 = 82689
Step 130: 82689 + 641 = 83330
Step 131: 83330 + 641 = 83971
Step 132: 83971 + 641 = 84612
Step 133: 84612 + 641 = 85253
Step 134: 85253 + 641 = 85894
Step 135: 85894 + 641 = 86535
Step 136: 86535 + 641 = 87176
Step 137: 87176 + 641 = 87817
Step 138: 87817 + 641 = 88458
Step 139: 88458 + 641 = 89099
Step 140: 89099 + 641 = 89740
Step 141: 89740 + 641 = 90381
Step 142: 90381 + 641 = 91022
Step 143: 91022 + 641 = 91663
Step 144: 91663 + 641 = 92304
Step 145: 92304 + 641 = 92945
Step 146: 92945 + 641 = 93586
Step 147: 93586 + 641 = 94227
Step 148: 94227 + 641 = 94868
Step 149: 94868 + 641 = 95509
Step 150: 95509 + 641 = 96150
Step 151: 96150 + 641 = 96791
Step 152: 96791 + 641 = 97432
Step 153: 97432 + 641 = 98073
Step 154: 98073 + 641 = 98714
Step 155: 98714 + 641 = 99355
Step 156: 99355 + 641 = 99996
Step 157: 99996 + 641 = 100637
Step 158: 100637 + 641 = 101278

So, 641*158 = 101278