OK, let's crack this. We're given 791 and our task is to multiply it by 90. Essentially, we'll be adding 791 to itself 90 times.
Step 1: 0 + 791 = 791
Step 2: 791 + 791 = 1582
Step 3: 1582 + 791 = 2373
Step 4: 2373 + 791 = 3164
Step 5: 3164 + 791 = 3955
Step 6: 3955 + 791 = 4746
Step 7: 4746 + 791 = 5537
Step 8: 5537 + 791 = 6328
Step 9: 6328 + 791 = 7119
Step 10: 7119 + 791 = 7910
Step 11: 7910 + 791 = 8701
Step 12: 8701 + 791 = 9492
Step 13: 9492 + 791 = 10283
Step 14: 10283 + 791 = 11074
Step 15: 11074 + 791 = 11865
Step 16: 11865 + 791 = 12656
Step 17: 12656 + 791 = 13447
Step 18: 13447 + 791 = 14238
Step 19: 14238 + 791 = 15029
Step 20: 15029 + 791 = 15820
Step 21: 15820 + 791 = 16611
Step 22: 16611 + 791 = 17402
Step 23: 17402 + 791 = 18193
Step 24: 18193 + 791 = 18984
Step 25: 18984 + 791 = 19775
Step 26: 19775 + 791 = 20566
Step 27: 20566 + 791 = 21357
Step 28: 21357 + 791 = 22148
Step 29: 22148 + 791 = 22939
Step 30: 22939 + 791 = 23730
Step 31: 23730 + 791 = 24521
Step 32: 24521 + 791 = 25312
Step 33: 25312 + 791 = 26103
Step 34: 26103 + 791 = 26894
Step 35: 26894 + 791 = 27685
Step 36: 27685 + 791 = 28476
Step 37: 28476 + 791 = 29267
Step 38: 29267 + 791 = 30058
Step 39: 30058 + 791 = 30849
Step 40: 30849 + 791 = 31640
Step 41: 31640 + 791 = 32431
Step 42: 32431 + 791 = 33222
Step 43: 33222 + 791 = 34013
Step 44: 34013 + 791 = 34804
Step 45: 34804 + 791 = 35595
Step 46: 35595 + 791 = 36386
Step 47: 36386 + 791 = 37177
Step 48: 37177 + 791 = 37968
Step 49: 37968 + 791 = 38759
Step 50: 38759 + 791 = 39550
Step 51: 39550 + 791 = 40341
Step 52: 40341 + 791 = 41132
Step 53: 41132 + 791 = 41923
Step 54: 41923 + 791 = 42714
Step 55: 42714 + 791 = 43505
Step 56: 43505 + 791 = 44296
Step 57: 44296 + 791 = 45087
Step 58: 45087 + 791 = 45878
Step 59: 45878 + 791 = 46669
Step 60: 46669 + 791 = 47460
Step 61: 47460 + 791 = 48251
Step 62: 48251 + 791 = 49042
Step 63: 49042 + 791 = 49833
Step 64: 49833 + 791 = 50624
Step 65: 50624 + 791 = 51415
Step 66: 51415 + 791 = 52206
Step 67: 52206 + 791 = 52997
Step 68: 52997 + 791 = 53788
Step 69: 53788 + 791 = 54579
Step 70: 54579 + 791 = 55370
Step 71: 55370 + 791 = 56161
Step 72: 56161 + 791 = 56952
Step 73: 56952 + 791 = 57743
Step 74: 57743 + 791 = 58534
Step 75: 58534 + 791 = 59325
Step 76: 59325 + 791 = 60116
Step 77: 60116 + 791 = 60907
Step 78: 60907 + 791 = 61698
Step 79: 61698 + 791 = 62489
Step 80: 62489 + 791 = 63280
Step 81: 63280 + 791 = 64071
Step 82: 64071 + 791 = 64862
Step 83: 64862 + 791 = 65653
Step 84: 65653 + 791 = 66444
Step 85: 66444 + 791 = 67235
Step 86: 67235 + 791 = 68026
Step 87: 68026 + 791 = 68817
Step 88: 68817 + 791 = 69608
Step 89: 69608 + 791 = 70399
Step 90: 70399 + 791 = 71190

So, 791*90 = 71190