Let's roll up our sleeves and solve this. We have 420 and we're going to multiply it by 48, essentially adding 420 to itself 48 times.
Step 1: 0 + 420 = 420
Step 2: 420 + 420 = 840
Step 3: 840 + 420 = 1260
Step 4: 1260 + 420 = 1680
Step 5: 1680 + 420 = 2100
Step 6: 2100 + 420 = 2520
Step 7: 2520 + 420 = 2940
Step 8: 2940 + 420 = 3360
Step 9: 3360 + 420 = 3780
Step 10: 3780 + 420 = 4200
Step 11: 4200 + 420 = 4620
Step 12: 4620 + 420 = 5040
Step 13: 5040 + 420 = 5460
Step 14: 5460 + 420 = 5880
Step 15: 5880 + 420 = 6300
Step 16: 6300 + 420 = 6720
Step 17: 6720 + 420 = 7140
Step 18: 7140 + 420 = 7560
Step 19: 7560 + 420 = 7980
Step 20: 7980 + 420 = 8400
Step 21: 8400 + 420 = 8820
Step 22: 8820 + 420 = 9240
Step 23: 9240 + 420 = 9660
Step 24: 9660 + 420 = 10080
Step 25: 10080 + 420 = 10500
Step 26: 10500 + 420 = 10920
Step 27: 10920 + 420 = 11340
Step 28: 11340 + 420 = 11760
Step 29: 11760 + 420 = 12180
Step 30: 12180 + 420 = 12600
Step 31: 12600 + 420 = 13020
Step 32: 13020 + 420 = 13440
Step 33: 13440 + 420 = 13860
Step 34: 13860 + 420 = 14280
Step 35: 14280 + 420 = 14700
Step 36: 14700 + 420 = 15120
Step 37: 15120 + 420 = 15540
Step 38: 15540 + 420 = 15960
Step 39: 15960 + 420 = 16380
Step 40: 16380 + 420 = 16800
Step 41: 16800 + 420 = 17220
Step 42: 17220 + 420 = 17640
Step 43: 17640 + 420 = 18060
Step 44: 18060 + 420 = 18480
Step 45: 18480 + 420 = 18900
Step 46: 18900 + 420 = 19320
Step 47: 19320 + 420 = 19740
Step 48: 19740 + 420 = 20160

So, 420*48 = 20160