Let's roll up our sleeves and solve this. We have 503 and we're going to multiply it by 40, essentially adding 503 to itself 40 times.
Step 1: 0 + 503 = 503
Step 2: 503 + 503 = 1006
Step 3: 1006 + 503 = 1509
Step 4: 1509 + 503 = 2012
Step 5: 2012 + 503 = 2515
Step 6: 2515 + 503 = 3018
Step 7: 3018 + 503 = 3521
Step 8: 3521 + 503 = 4024
Step 9: 4024 + 503 = 4527
Step 10: 4527 + 503 = 5030
Step 11: 5030 + 503 = 5533
Step 12: 5533 + 503 = 6036
Step 13: 6036 + 503 = 6539
Step 14: 6539 + 503 = 7042
Step 15: 7042 + 503 = 7545
Step 16: 7545 + 503 = 8048
Step 17: 8048 + 503 = 8551
Step 18: 8551 + 503 = 9054
Step 19: 9054 + 503 = 9557
Step 20: 9557 + 503 = 10060
Step 21: 10060 + 503 = 10563
Step 22: 10563 + 503 = 11066
Step 23: 11066 + 503 = 11569
Step 24: 11569 + 503 = 12072
Step 25: 12072 + 503 = 12575
Step 26: 12575 + 503 = 13078
Step 27: 13078 + 503 = 13581
Step 28: 13581 + 503 = 14084
Step 29: 14084 + 503 = 14587
Step 30: 14587 + 503 = 15090
Step 31: 15090 + 503 = 15593
Step 32: 15593 + 503 = 16096
Step 33: 16096 + 503 = 16599
Step 34: 16599 + 503 = 17102
Step 35: 17102 + 503 = 17605
Step 36: 17605 + 503 = 18108
Step 37: 18108 + 503 = 18611
Step 38: 18611 + 503 = 19114
Step 39: 19114 + 503 = 19617
Step 40: 19617 + 503 = 20120

So, 503*40 = 20120