OK, let's crack this. We're given 770 and our task is to multiply it by 91. Essentially, we'll be adding 770 to itself 91 times.
Step 1: 0 + 770 = 770
Step 2: 770 + 770 = 1540
Step 3: 1540 + 770 = 2310
Step 4: 2310 + 770 = 3080
Step 5: 3080 + 770 = 3850
Step 6: 3850 + 770 = 4620
Step 7: 4620 + 770 = 5390
Step 8: 5390 + 770 = 6160
Step 9: 6160 + 770 = 6930
Step 10: 6930 + 770 = 7700
Step 11: 7700 + 770 = 8470
Step 12: 8470 + 770 = 9240
Step 13: 9240 + 770 = 10010
Step 14: 10010 + 770 = 10780
Step 15: 10780 + 770 = 11550
Step 16: 11550 + 770 = 12320
Step 17: 12320 + 770 = 13090
Step 18: 13090 + 770 = 13860
Step 19: 13860 + 770 = 14630
Step 20: 14630 + 770 = 15400
Step 21: 15400 + 770 = 16170
Step 22: 16170 + 770 = 16940
Step 23: 16940 + 770 = 17710
Step 24: 17710 + 770 = 18480
Step 25: 18480 + 770 = 19250
Step 26: 19250 + 770 = 20020
Step 27: 20020 + 770 = 20790
Step 28: 20790 + 770 = 21560
Step 29: 21560 + 770 = 22330
Step 30: 22330 + 770 = 23100
Step 31: 23100 + 770 = 23870
Step 32: 23870 + 770 = 24640
Step 33: 24640 + 770 = 25410
Step 34: 25410 + 770 = 26180
Step 35: 26180 + 770 = 26950
Step 36: 26950 + 770 = 27720
Step 37: 27720 + 770 = 28490
Step 38: 28490 + 770 = 29260
Step 39: 29260 + 770 = 30030
Step 40: 30030 + 770 = 30800
Step 41: 30800 + 770 = 31570
Step 42: 31570 + 770 = 32340
Step 43: 32340 + 770 = 33110
Step 44: 33110 + 770 = 33880
Step 45: 33880 + 770 = 34650
Step 46: 34650 + 770 = 35420
Step 47: 35420 + 770 = 36190
Step 48: 36190 + 770 = 36960
Step 49: 36960 + 770 = 37730
Step 50: 37730 + 770 = 38500
Step 51: 38500 + 770 = 39270
Step 52: 39270 + 770 = 40040
Step 53: 40040 + 770 = 40810
Step 54: 40810 + 770 = 41580
Step 55: 41580 + 770 = 42350
Step 56: 42350 + 770 = 43120
Step 57: 43120 + 770 = 43890
Step 58: 43890 + 770 = 44660
Step 59: 44660 + 770 = 45430
Step 60: 45430 + 770 = 46200
Step 61: 46200 + 770 = 46970
Step 62: 46970 + 770 = 47740
Step 63: 47740 + 770 = 48510
Step 64: 48510 + 770 = 49280
Step 65: 49280 + 770 = 50050
Step 66: 50050 + 770 = 50820
Step 67: 50820 + 770 = 51590
Step 68: 51590 + 770 = 52360
Step 69: 52360 + 770 = 53130
Step 70: 53130 + 770 = 53900
Step 71: 53900 + 770 = 54670
Step 72: 54670 + 770 = 55440
Step 73: 55440 + 770 = 56210
Step 74: 56210 + 770 = 56980
Step 75: 56980 + 770 = 57750
Step 76: 57750 + 770 = 58520
Step 77: 58520 + 770 = 59290
Step 78: 59290 + 770 = 60060
Step 79: 60060 + 770 = 60830
Step 80: 60830 + 770 = 61600
Step 81: 61600 + 770 = 62370
Step 82: 62370 + 770 = 63140
Step 83: 63140 + 770 = 63910
Step 84: 63910 + 770 = 64680
Step 85: 64680 + 770 = 65450
Step 86: 65450 + 770 = 66220
Step 87: 66220 + 770 = 66990
Step 88: 66990 + 770 = 67760
Step 89: 67760 + 770 = 68530
Step 90: 68530 + 770 = 69300
Step 91: 69300 + 770 = 70070

So, 770*91 = 70070