OK, let's crack this. We're given 873 and our task is to multiply it by 90. Essentially, we'll be adding 873 to itself 90 times.
Step 1: 0 + 873 = 873
Step 2: 873 + 873 = 1746
Step 3: 1746 + 873 = 2619
Step 4: 2619 + 873 = 3492
Step 5: 3492 + 873 = 4365
Step 6: 4365 + 873 = 5238
Step 7: 5238 + 873 = 6111
Step 8: 6111 + 873 = 6984
Step 9: 6984 + 873 = 7857
Step 10: 7857 + 873 = 8730
Step 11: 8730 + 873 = 9603
Step 12: 9603 + 873 = 10476
Step 13: 10476 + 873 = 11349
Step 14: 11349 + 873 = 12222
Step 15: 12222 + 873 = 13095
Step 16: 13095 + 873 = 13968
Step 17: 13968 + 873 = 14841
Step 18: 14841 + 873 = 15714
Step 19: 15714 + 873 = 16587
Step 20: 16587 + 873 = 17460
Step 21: 17460 + 873 = 18333
Step 22: 18333 + 873 = 19206
Step 23: 19206 + 873 = 20079
Step 24: 20079 + 873 = 20952
Step 25: 20952 + 873 = 21825
Step 26: 21825 + 873 = 22698
Step 27: 22698 + 873 = 23571
Step 28: 23571 + 873 = 24444
Step 29: 24444 + 873 = 25317
Step 30: 25317 + 873 = 26190
Step 31: 26190 + 873 = 27063
Step 32: 27063 + 873 = 27936
Step 33: 27936 + 873 = 28809
Step 34: 28809 + 873 = 29682
Step 35: 29682 + 873 = 30555
Step 36: 30555 + 873 = 31428
Step 37: 31428 + 873 = 32301
Step 38: 32301 + 873 = 33174
Step 39: 33174 + 873 = 34047
Step 40: 34047 + 873 = 34920
Step 41: 34920 + 873 = 35793
Step 42: 35793 + 873 = 36666
Step 43: 36666 + 873 = 37539
Step 44: 37539 + 873 = 38412
Step 45: 38412 + 873 = 39285
Step 46: 39285 + 873 = 40158
Step 47: 40158 + 873 = 41031
Step 48: 41031 + 873 = 41904
Step 49: 41904 + 873 = 42777
Step 50: 42777 + 873 = 43650
Step 51: 43650 + 873 = 44523
Step 52: 44523 + 873 = 45396
Step 53: 45396 + 873 = 46269
Step 54: 46269 + 873 = 47142
Step 55: 47142 + 873 = 48015
Step 56: 48015 + 873 = 48888
Step 57: 48888 + 873 = 49761
Step 58: 49761 + 873 = 50634
Step 59: 50634 + 873 = 51507
Step 60: 51507 + 873 = 52380
Step 61: 52380 + 873 = 53253
Step 62: 53253 + 873 = 54126
Step 63: 54126 + 873 = 54999
Step 64: 54999 + 873 = 55872
Step 65: 55872 + 873 = 56745
Step 66: 56745 + 873 = 57618
Step 67: 57618 + 873 = 58491
Step 68: 58491 + 873 = 59364
Step 69: 59364 + 873 = 60237
Step 70: 60237 + 873 = 61110
Step 71: 61110 + 873 = 61983
Step 72: 61983 + 873 = 62856
Step 73: 62856 + 873 = 63729
Step 74: 63729 + 873 = 64602
Step 75: 64602 + 873 = 65475
Step 76: 65475 + 873 = 66348
Step 77: 66348 + 873 = 67221
Step 78: 67221 + 873 = 68094
Step 79: 68094 + 873 = 68967
Step 80: 68967 + 873 = 69840
Step 81: 69840 + 873 = 70713
Step 82: 70713 + 873 = 71586
Step 83: 71586 + 873 = 72459
Step 84: 72459 + 873 = 73332
Step 85: 73332 + 873 = 74205
Step 86: 74205 + 873 = 75078
Step 87: 75078 + 873 = 75951
Step 88: 75951 + 873 = 76824
Step 89: 76824 + 873 = 77697
Step 90: 77697 + 873 = 78570

So, 873*90 = 78570