OK, let's crack this. We're given 640 and our task is to multiply it by 126. Essentially, we'll be adding 640 to itself 126 times.
Step 1: 0 + 640 = 640
Step 2: 640 + 640 = 1280
Step 3: 1280 + 640 = 1920
Step 4: 1920 + 640 = 2560
Step 5: 2560 + 640 = 3200
Step 6: 3200 + 640 = 3840
Step 7: 3840 + 640 = 4480
Step 8: 4480 + 640 = 5120
Step 9: 5120 + 640 = 5760
Step 10: 5760 + 640 = 6400
Step 11: 6400 + 640 = 7040
Step 12: 7040 + 640 = 7680
Step 13: 7680 + 640 = 8320
Step 14: 8320 + 640 = 8960
Step 15: 8960 + 640 = 9600
Step 16: 9600 + 640 = 10240
Step 17: 10240 + 640 = 10880
Step 18: 10880 + 640 = 11520
Step 19: 11520 + 640 = 12160
Step 20: 12160 + 640 = 12800
Step 21: 12800 + 640 = 13440
Step 22: 13440 + 640 = 14080
Step 23: 14080 + 640 = 14720
Step 24: 14720 + 640 = 15360
Step 25: 15360 + 640 = 16000
Step 26: 16000 + 640 = 16640
Step 27: 16640 + 640 = 17280
Step 28: 17280 + 640 = 17920
Step 29: 17920 + 640 = 18560
Step 30: 18560 + 640 = 19200
Step 31: 19200 + 640 = 19840
Step 32: 19840 + 640 = 20480
Step 33: 20480 + 640 = 21120
Step 34: 21120 + 640 = 21760
Step 35: 21760 + 640 = 22400
Step 36: 22400 + 640 = 23040
Step 37: 23040 + 640 = 23680
Step 38: 23680 + 640 = 24320
Step 39: 24320 + 640 = 24960
Step 40: 24960 + 640 = 25600
Step 41: 25600 + 640 = 26240
Step 42: 26240 + 640 = 26880
Step 43: 26880 + 640 = 27520
Step 44: 27520 + 640 = 28160
Step 45: 28160 + 640 = 28800
Step 46: 28800 + 640 = 29440
Step 47: 29440 + 640 = 30080
Step 48: 30080 + 640 = 30720
Step 49: 30720 + 640 = 31360
Step 50: 31360 + 640 = 32000
Step 51: 32000 + 640 = 32640
Step 52: 32640 + 640 = 33280
Step 53: 33280 + 640 = 33920
Step 54: 33920 + 640 = 34560
Step 55: 34560 + 640 = 35200
Step 56: 35200 + 640 = 35840
Step 57: 35840 + 640 = 36480
Step 58: 36480 + 640 = 37120
Step 59: 37120 + 640 = 37760
Step 60: 37760 + 640 = 38400
Step 61: 38400 + 640 = 39040
Step 62: 39040 + 640 = 39680
Step 63: 39680 + 640 = 40320
Step 64: 40320 + 640 = 40960
Step 65: 40960 + 640 = 41600
Step 66: 41600 + 640 = 42240
Step 67: 42240 + 640 = 42880
Step 68: 42880 + 640 = 43520
Step 69: 43520 + 640 = 44160
Step 70: 44160 + 640 = 44800
Step 71: 44800 + 640 = 45440
Step 72: 45440 + 640 = 46080
Step 73: 46080 + 640 = 46720
Step 74: 46720 + 640 = 47360
Step 75: 47360 + 640 = 48000
Step 76: 48000 + 640 = 48640
Step 77: 48640 + 640 = 49280
Step 78: 49280 + 640 = 49920
Step 79: 49920 + 640 = 50560
Step 80: 50560 + 640 = 51200
Step 81: 51200 + 640 = 51840
Step 82: 51840 + 640 = 52480
Step 83: 52480 + 640 = 53120
Step 84: 53120 + 640 = 53760
Step 85: 53760 + 640 = 54400
Step 86: 54400 + 640 = 55040
Step 87: 55040 + 640 = 55680
Step 88: 55680 + 640 = 56320
Step 89: 56320 + 640 = 56960
Step 90: 56960 + 640 = 57600
Step 91: 57600 + 640 = 58240
Step 92: 58240 + 640 = 58880
Step 93: 58880 + 640 = 59520
Step 94: 59520 + 640 = 60160
Step 95: 60160 + 640 = 60800
Step 96: 60800 + 640 = 61440
Step 97: 61440 + 640 = 62080
Step 98: 62080 + 640 = 62720
Step 99: 62720 + 640 = 63360
Step 100: 63360 + 640 = 64000
Step 101: 64000 + 640 = 64640
Step 102: 64640 + 640 = 65280
Step 103: 65280 + 640 = 65920
Step 104: 65920 + 640 = 66560
Step 105: 66560 + 640 = 67200
Step 106: 67200 + 640 = 67840
Step 107: 67840 + 640 = 68480
Step 108: 68480 + 640 = 69120
Step 109: 69120 + 640 = 69760
Step 110: 69760 + 640 = 70400
Step 111: 70400 + 640 = 71040
Step 112: 71040 + 640 = 71680
Step 113: 71680 + 640 = 72320
Step 114: 72320 + 640 = 72960
Step 115: 72960 + 640 = 73600
Step 116: 73600 + 640 = 74240
Step 117: 74240 + 640 = 74880
Step 118: 74880 + 640 = 75520
Step 119: 75520 + 640 = 76160
Step 120: 76160 + 640 = 76800
Step 121: 76800 + 640 = 77440
Step 122: 77440 + 640 = 78080
Step 123: 78080 + 640 = 78720
Step 124: 78720 + 640 = 79360
Step 125: 79360 + 640 = 80000
Step 126: 80000 + 640 = 80640

So, 640*126 = 80640