OK, let's work through this together. We're starting with 783 and we're going to multiply it by 90, which means we add 783 to itself 90 times.
Step 1: 0 + 783 = 783
Step 2: 783 + 783 = 1566
Step 3: 1566 + 783 = 2349
Step 4: 2349 + 783 = 3132
Step 5: 3132 + 783 = 3915
Step 6: 3915 + 783 = 4698
Step 7: 4698 + 783 = 5481
Step 8: 5481 + 783 = 6264
Step 9: 6264 + 783 = 7047
Step 10: 7047 + 783 = 7830
Step 11: 7830 + 783 = 8613
Step 12: 8613 + 783 = 9396
Step 13: 9396 + 783 = 10179
Step 14: 10179 + 783 = 10962
Step 15: 10962 + 783 = 11745
Step 16: 11745 + 783 = 12528
Step 17: 12528 + 783 = 13311
Step 18: 13311 + 783 = 14094
Step 19: 14094 + 783 = 14877
Step 20: 14877 + 783 = 15660
Step 21: 15660 + 783 = 16443
Step 22: 16443 + 783 = 17226
Step 23: 17226 + 783 = 18009
Step 24: 18009 + 783 = 18792
Step 25: 18792 + 783 = 19575
Step 26: 19575 + 783 = 20358
Step 27: 20358 + 783 = 21141
Step 28: 21141 + 783 = 21924
Step 29: 21924 + 783 = 22707
Step 30: 22707 + 783 = 23490
Step 31: 23490 + 783 = 24273
Step 32: 24273 + 783 = 25056
Step 33: 25056 + 783 = 25839
Step 34: 25839 + 783 = 26622
Step 35: 26622 + 783 = 27405
Step 36: 27405 + 783 = 28188
Step 37: 28188 + 783 = 28971
Step 38: 28971 + 783 = 29754
Step 39: 29754 + 783 = 30537
Step 40: 30537 + 783 = 31320
Step 41: 31320 + 783 = 32103
Step 42: 32103 + 783 = 32886
Step 43: 32886 + 783 = 33669
Step 44: 33669 + 783 = 34452
Step 45: 34452 + 783 = 35235
Step 46: 35235 + 783 = 36018
Step 47: 36018 + 783 = 36801
Step 48: 36801 + 783 = 37584
Step 49: 37584 + 783 = 38367
Step 50: 38367 + 783 = 39150
Step 51: 39150 + 783 = 39933
Step 52: 39933 + 783 = 40716
Step 53: 40716 + 783 = 41499
Step 54: 41499 + 783 = 42282
Step 55: 42282 + 783 = 43065
Step 56: 43065 + 783 = 43848
Step 57: 43848 + 783 = 44631
Step 58: 44631 + 783 = 45414
Step 59: 45414 + 783 = 46197
Step 60: 46197 + 783 = 46980
Step 61: 46980 + 783 = 47763
Step 62: 47763 + 783 = 48546
Step 63: 48546 + 783 = 49329
Step 64: 49329 + 783 = 50112
Step 65: 50112 + 783 = 50895
Step 66: 50895 + 783 = 51678
Step 67: 51678 + 783 = 52461
Step 68: 52461 + 783 = 53244
Step 69: 53244 + 783 = 54027
Step 70: 54027 + 783 = 54810
Step 71: 54810 + 783 = 55593
Step 72: 55593 + 783 = 56376
Step 73: 56376 + 783 = 57159
Step 74: 57159 + 783 = 57942
Step 75: 57942 + 783 = 58725
Step 76: 58725 + 783 = 59508
Step 77: 59508 + 783 = 60291
Step 78: 60291 + 783 = 61074
Step 79: 61074 + 783 = 61857
Step 80: 61857 + 783 = 62640
Step 81: 62640 + 783 = 63423
Step 82: 63423 + 783 = 64206
Step 83: 64206 + 783 = 64989
Step 84: 64989 + 783 = 65772
Step 85: 65772 + 783 = 66555
Step 86: 66555 + 783 = 67338
Step 87: 67338 + 783 = 68121
Step 88: 68121 + 783 = 68904
Step 89: 68904 + 783 = 69687
Step 90: 69687 + 783 = 70470

So, 783*90 = 70470