Sure thing, let's get straight to it. We start with 641 and we're going to multiply it by 90, which means adding 641 to itself 90 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

So, 641*90 = 57690