OK, let's work through this together. We're starting with 400 and we're going to multiply it by 121, which means we add 400 to itself 121 times.
Step 1: 0 + 400 = 400
Step 2: 400 + 400 = 800
Step 3: 800 + 400 = 1200
Step 4: 1200 + 400 = 1600
Step 5: 1600 + 400 = 2000
Step 6: 2000 + 400 = 2400
Step 7: 2400 + 400 = 2800
Step 8: 2800 + 400 = 3200
Step 9: 3200 + 400 = 3600
Step 10: 3600 + 400 = 4000
Step 11: 4000 + 400 = 4400
Step 12: 4400 + 400 = 4800
Step 13: 4800 + 400 = 5200
Step 14: 5200 + 400 = 5600
Step 15: 5600 + 400 = 6000
Step 16: 6000 + 400 = 6400
Step 17: 6400 + 400 = 6800
Step 18: 6800 + 400 = 7200
Step 19: 7200 + 400 = 7600
Step 20: 7600 + 400 = 8000
Step 21: 8000 + 400 = 8400
Step 22: 8400 + 400 = 8800
Step 23: 8800 + 400 = 9200
Step 24: 9200 + 400 = 9600
Step 25: 9600 + 400 = 10000
Step 26: 10000 + 400 = 10400
Step 27: 10400 + 400 = 10800
Step 28: 10800 + 400 = 11200
Step 29: 11200 + 400 = 11600
Step 30: 11600 + 400 = 12000
Step 31: 12000 + 400 = 12400
Step 32: 12400 + 400 = 12800
Step 33: 12800 + 400 = 13200
Step 34: 13200 + 400 = 13600
Step 35: 13600 + 400 = 14000
Step 36: 14000 + 400 = 14400
Step 37: 14400 + 400 = 14800
Step 38: 14800 + 400 = 15200
Step 39: 15200 + 400 = 15600
Step 40: 15600 + 400 = 16000
Step 41: 16000 + 400 = 16400
Step 42: 16400 + 400 = 16800
Step 43: 16800 + 400 = 17200
Step 44: 17200 + 400 = 17600
Step 45: 17600 + 400 = 18000
Step 46: 18000 + 400 = 18400
Step 47: 18400 + 400 = 18800
Step 48: 18800 + 400 = 19200
Step 49: 19200 + 400 = 19600
Step 50: 19600 + 400 = 20000
Step 51: 20000 + 400 = 20400
Step 52: 20400 + 400 = 20800
Step 53: 20800 + 400 = 21200
Step 54: 21200 + 400 = 21600
Step 55: 21600 + 400 = 22000
Step 56: 22000 + 400 = 22400
Step 57: 22400 + 400 = 22800
Step 58: 22800 + 400 = 23200
Step 59: 23200 + 400 = 23600
Step 60: 23600 + 400 = 24000
Step 61: 24000 + 400 = 24400
Step 62: 24400 + 400 = 24800
Step 63: 24800 + 400 = 25200
Step 64: 25200 + 400 = 25600
Step 65: 25600 + 400 = 26000
Step 66: 26000 + 400 = 26400
Step 67: 26400 + 400 = 26800
Step 68: 26800 + 400 = 27200
Step 69: 27200 + 400 = 27600
Step 70: 27600 + 400 = 28000
Step 71: 28000 + 400 = 28400
Step 72: 28400 + 400 = 28800
Step 73: 28800 + 400 = 29200
Step 74: 29200 + 400 = 29600
Step 75: 29600 + 400 = 30000
Step 76: 30000 + 400 = 30400
Step 77: 30400 + 400 = 30800
Step 78: 30800 + 400 = 31200
Step 79: 31200 + 400 = 31600
Step 80: 31600 + 400 = 32000
Step 81: 32000 + 400 = 32400
Step 82: 32400 + 400 = 32800
Step 83: 32800 + 400 = 33200
Step 84: 33200 + 400 = 33600
Step 85: 33600 + 400 = 34000
Step 86: 34000 + 400 = 34400
Step 87: 34400 + 400 = 34800
Step 88: 34800 + 400 = 35200
Step 89: 35200 + 400 = 35600
Step 90: 35600 + 400 = 36000
Step 91: 36000 + 400 = 36400
Step 92: 36400 + 400 = 36800
Step 93: 36800 + 400 = 37200
Step 94: 37200 + 400 = 37600
Step 95: 37600 + 400 = 38000
Step 96: 38000 + 400 = 38400
Step 97: 38400 + 400 = 38800
Step 98: 38800 + 400 = 39200
Step 99: 39200 + 400 = 39600
Step 100: 39600 + 400 = 40000
Step 101: 40000 + 400 = 40400
Step 102: 40400 + 400 = 40800
Step 103: 40800 + 400 = 41200
Step 104: 41200 + 400 = 41600
Step 105: 41600 + 400 = 42000
Step 106: 42000 + 400 = 42400
Step 107: 42400 + 400 = 42800
Step 108: 42800 + 400 = 43200
Step 109: 43200 + 400 = 43600
Step 110: 43600 + 400 = 44000
Step 111: 44000 + 400 = 44400
Step 112: 44400 + 400 = 44800
Step 113: 44800 + 400 = 45200
Step 114: 45200 + 400 = 45600
Step 115: 45600 + 400 = 46000
Step 116: 46000 + 400 = 46400
Step 117: 46400 + 400 = 46800
Step 118: 46800 + 400 = 47200
Step 119: 47200 + 400 = 47600
Step 120: 47600 + 400 = 48000
Step 121: 48000 + 400 = 48400

So, 400*121 = 48400