OK, let's crack this. We're given 900 and our task is to multiply it by 86. Essentially, we'll be adding 900 to itself 86 times.
Step 1: 0 + 900 = 900
Step 2: 900 + 900 = 1800
Step 3: 1800 + 900 = 2700
Step 4: 2700 + 900 = 3600
Step 5: 3600 + 900 = 4500
Step 6: 4500 + 900 = 5400
Step 7: 5400 + 900 = 6300
Step 8: 6300 + 900 = 7200
Step 9: 7200 + 900 = 8100
Step 10: 8100 + 900 = 9000
Step 11: 9000 + 900 = 9900
Step 12: 9900 + 900 = 10800
Step 13: 10800 + 900 = 11700
Step 14: 11700 + 900 = 12600
Step 15: 12600 + 900 = 13500
Step 16: 13500 + 900 = 14400
Step 17: 14400 + 900 = 15300
Step 18: 15300 + 900 = 16200
Step 19: 16200 + 900 = 17100
Step 20: 17100 + 900 = 18000
Step 21: 18000 + 900 = 18900
Step 22: 18900 + 900 = 19800
Step 23: 19800 + 900 = 20700
Step 24: 20700 + 900 = 21600
Step 25: 21600 + 900 = 22500
Step 26: 22500 + 900 = 23400
Step 27: 23400 + 900 = 24300
Step 28: 24300 + 900 = 25200
Step 29: 25200 + 900 = 26100
Step 30: 26100 + 900 = 27000
Step 31: 27000 + 900 = 27900
Step 32: 27900 + 900 = 28800
Step 33: 28800 + 900 = 29700
Step 34: 29700 + 900 = 30600
Step 35: 30600 + 900 = 31500
Step 36: 31500 + 900 = 32400
Step 37: 32400 + 900 = 33300
Step 38: 33300 + 900 = 34200
Step 39: 34200 + 900 = 35100
Step 40: 35100 + 900 = 36000
Step 41: 36000 + 900 = 36900
Step 42: 36900 + 900 = 37800
Step 43: 37800 + 900 = 38700
Step 44: 38700 + 900 = 39600
Step 45: 39600 + 900 = 40500
Step 46: 40500 + 900 = 41400
Step 47: 41400 + 900 = 42300
Step 48: 42300 + 900 = 43200
Step 49: 43200 + 900 = 44100
Step 50: 44100 + 900 = 45000
Step 51: 45000 + 900 = 45900
Step 52: 45900 + 900 = 46800
Step 53: 46800 + 900 = 47700
Step 54: 47700 + 900 = 48600
Step 55: 48600 + 900 = 49500
Step 56: 49500 + 900 = 50400
Step 57: 50400 + 900 = 51300
Step 58: 51300 + 900 = 52200
Step 59: 52200 + 900 = 53100
Step 60: 53100 + 900 = 54000
Step 61: 54000 + 900 = 54900
Step 62: 54900 + 900 = 55800
Step 63: 55800 + 900 = 56700
Step 64: 56700 + 900 = 57600
Step 65: 57600 + 900 = 58500
Step 66: 58500 + 900 = 59400
Step 67: 59400 + 900 = 60300
Step 68: 60300 + 900 = 61200
Step 69: 61200 + 900 = 62100
Step 70: 62100 + 900 = 63000
Step 71: 63000 + 900 = 63900
Step 72: 63900 + 900 = 64800
Step 73: 64800 + 900 = 65700
Step 74: 65700 + 900 = 66600
Step 75: 66600 + 900 = 67500
Step 76: 67500 + 900 = 68400
Step 77: 68400 + 900 = 69300
Step 78: 69300 + 900 = 70200
Step 79: 70200 + 900 = 71100
Step 80: 71100 + 900 = 72000
Step 81: 72000 + 900 = 72900
Step 82: 72900 + 900 = 73800
Step 83: 73800 + 900 = 74700
Step 84: 74700 + 900 = 75600
Step 85: 75600 + 900 = 76500
Step 86: 76500 + 900 = 77400

So, 900*86 = 77400