Let's roll up our sleeves and solve this. We have 681 and we're going to multiply it by 90, essentially adding 681 to itself 90 times.
Step 1: 0 + 681 = 681
Step 2: 681 + 681 = 1362
Step 3: 1362 + 681 = 2043
Step 4: 2043 + 681 = 2724
Step 5: 2724 + 681 = 3405
Step 6: 3405 + 681 = 4086
Step 7: 4086 + 681 = 4767
Step 8: 4767 + 681 = 5448
Step 9: 5448 + 681 = 6129
Step 10: 6129 + 681 = 6810
Step 11: 6810 + 681 = 7491
Step 12: 7491 + 681 = 8172
Step 13: 8172 + 681 = 8853
Step 14: 8853 + 681 = 9534
Step 15: 9534 + 681 = 10215
Step 16: 10215 + 681 = 10896
Step 17: 10896 + 681 = 11577
Step 18: 11577 + 681 = 12258
Step 19: 12258 + 681 = 12939
Step 20: 12939 + 681 = 13620
Step 21: 13620 + 681 = 14301
Step 22: 14301 + 681 = 14982
Step 23: 14982 + 681 = 15663
Step 24: 15663 + 681 = 16344
Step 25: 16344 + 681 = 17025
Step 26: 17025 + 681 = 17706
Step 27: 17706 + 681 = 18387
Step 28: 18387 + 681 = 19068
Step 29: 19068 + 681 = 19749
Step 30: 19749 + 681 = 20430
Step 31: 20430 + 681 = 21111
Step 32: 21111 + 681 = 21792
Step 33: 21792 + 681 = 22473
Step 34: 22473 + 681 = 23154
Step 35: 23154 + 681 = 23835
Step 36: 23835 + 681 = 24516
Step 37: 24516 + 681 = 25197
Step 38: 25197 + 681 = 25878
Step 39: 25878 + 681 = 26559
Step 40: 26559 + 681 = 27240
Step 41: 27240 + 681 = 27921
Step 42: 27921 + 681 = 28602
Step 43: 28602 + 681 = 29283
Step 44: 29283 + 681 = 29964
Step 45: 29964 + 681 = 30645
Step 46: 30645 + 681 = 31326
Step 47: 31326 + 681 = 32007
Step 48: 32007 + 681 = 32688
Step 49: 32688 + 681 = 33369
Step 50: 33369 + 681 = 34050
Step 51: 34050 + 681 = 34731
Step 52: 34731 + 681 = 35412
Step 53: 35412 + 681 = 36093
Step 54: 36093 + 681 = 36774
Step 55: 36774 + 681 = 37455
Step 56: 37455 + 681 = 38136
Step 57: 38136 + 681 = 38817
Step 58: 38817 + 681 = 39498
Step 59: 39498 + 681 = 40179
Step 60: 40179 + 681 = 40860
Step 61: 40860 + 681 = 41541
Step 62: 41541 + 681 = 42222
Step 63: 42222 + 681 = 42903
Step 64: 42903 + 681 = 43584
Step 65: 43584 + 681 = 44265
Step 66: 44265 + 681 = 44946
Step 67: 44946 + 681 = 45627
Step 68: 45627 + 681 = 46308
Step 69: 46308 + 681 = 46989
Step 70: 46989 + 681 = 47670
Step 71: 47670 + 681 = 48351
Step 72: 48351 + 681 = 49032
Step 73: 49032 + 681 = 49713
Step 74: 49713 + 681 = 50394
Step 75: 50394 + 681 = 51075
Step 76: 51075 + 681 = 51756
Step 77: 51756 + 681 = 52437
Step 78: 52437 + 681 = 53118
Step 79: 53118 + 681 = 53799
Step 80: 53799 + 681 = 54480
Step 81: 54480 + 681 = 55161
Step 82: 55161 + 681 = 55842
Step 83: 55842 + 681 = 56523
Step 84: 56523 + 681 = 57204
Step 85: 57204 + 681 = 57885
Step 86: 57885 + 681 = 58566
Step 87: 58566 + 681 = 59247
Step 88: 59247 + 681 = 59928
Step 89: 59928 + 681 = 60609
Step 90: 60609 + 681 = 61290

So, 681*90 = 61290