OK, let's work through this together. We're starting with 891 and we're going to multiply it by 80, which means we add 891 to itself 80 times.
Step 1: 0 + 891 = 891
Step 2: 891 + 891 = 1782
Step 3: 1782 + 891 = 2673
Step 4: 2673 + 891 = 3564
Step 5: 3564 + 891 = 4455
Step 6: 4455 + 891 = 5346
Step 7: 5346 + 891 = 6237
Step 8: 6237 + 891 = 7128
Step 9: 7128 + 891 = 8019
Step 10: 8019 + 891 = 8910
Step 11: 8910 + 891 = 9801
Step 12: 9801 + 891 = 10692
Step 13: 10692 + 891 = 11583
Step 14: 11583 + 891 = 12474
Step 15: 12474 + 891 = 13365
Step 16: 13365 + 891 = 14256
Step 17: 14256 + 891 = 15147
Step 18: 15147 + 891 = 16038
Step 19: 16038 + 891 = 16929
Step 20: 16929 + 891 = 17820
Step 21: 17820 + 891 = 18711
Step 22: 18711 + 891 = 19602
Step 23: 19602 + 891 = 20493
Step 24: 20493 + 891 = 21384
Step 25: 21384 + 891 = 22275
Step 26: 22275 + 891 = 23166
Step 27: 23166 + 891 = 24057
Step 28: 24057 + 891 = 24948
Step 29: 24948 + 891 = 25839
Step 30: 25839 + 891 = 26730
Step 31: 26730 + 891 = 27621
Step 32: 27621 + 891 = 28512
Step 33: 28512 + 891 = 29403
Step 34: 29403 + 891 = 30294
Step 35: 30294 + 891 = 31185
Step 36: 31185 + 891 = 32076
Step 37: 32076 + 891 = 32967
Step 38: 32967 + 891 = 33858
Step 39: 33858 + 891 = 34749
Step 40: 34749 + 891 = 35640
Step 41: 35640 + 891 = 36531
Step 42: 36531 + 891 = 37422
Step 43: 37422 + 891 = 38313
Step 44: 38313 + 891 = 39204
Step 45: 39204 + 891 = 40095
Step 46: 40095 + 891 = 40986
Step 47: 40986 + 891 = 41877
Step 48: 41877 + 891 = 42768
Step 49: 42768 + 891 = 43659
Step 50: 43659 + 891 = 44550
Step 51: 44550 + 891 = 45441
Step 52: 45441 + 891 = 46332
Step 53: 46332 + 891 = 47223
Step 54: 47223 + 891 = 48114
Step 55: 48114 + 891 = 49005
Step 56: 49005 + 891 = 49896
Step 57: 49896 + 891 = 50787
Step 58: 50787 + 891 = 51678
Step 59: 51678 + 891 = 52569
Step 60: 52569 + 891 = 53460
Step 61: 53460 + 891 = 54351
Step 62: 54351 + 891 = 55242
Step 63: 55242 + 891 = 56133
Step 64: 56133 + 891 = 57024
Step 65: 57024 + 891 = 57915
Step 66: 57915 + 891 = 58806
Step 67: 58806 + 891 = 59697
Step 68: 59697 + 891 = 60588
Step 69: 60588 + 891 = 61479
Step 70: 61479 + 891 = 62370
Step 71: 62370 + 891 = 63261
Step 72: 63261 + 891 = 64152
Step 73: 64152 + 891 = 65043
Step 74: 65043 + 891 = 65934
Step 75: 65934 + 891 = 66825
Step 76: 66825 + 891 = 67716
Step 77: 67716 + 891 = 68607
Step 78: 68607 + 891 = 69498
Step 79: 69498 + 891 = 70389
Step 80: 70389 + 891 = 71280

So, 891*80 = 71280