Let's roll up our sleeves and solve this. We have 831 and we're going to multiply it by 40, essentially adding 831 to itself 40 times.
Step 1: 0 + 831 = 831
Step 2: 831 + 831 = 1662
Step 3: 1662 + 831 = 2493
Step 4: 2493 + 831 = 3324
Step 5: 3324 + 831 = 4155
Step 6: 4155 + 831 = 4986
Step 7: 4986 + 831 = 5817
Step 8: 5817 + 831 = 6648
Step 9: 6648 + 831 = 7479
Step 10: 7479 + 831 = 8310
Step 11: 8310 + 831 = 9141
Step 12: 9141 + 831 = 9972
Step 13: 9972 + 831 = 10803
Step 14: 10803 + 831 = 11634
Step 15: 11634 + 831 = 12465
Step 16: 12465 + 831 = 13296
Step 17: 13296 + 831 = 14127
Step 18: 14127 + 831 = 14958
Step 19: 14958 + 831 = 15789
Step 20: 15789 + 831 = 16620
Step 21: 16620 + 831 = 17451
Step 22: 17451 + 831 = 18282
Step 23: 18282 + 831 = 19113
Step 24: 19113 + 831 = 19944
Step 25: 19944 + 831 = 20775
Step 26: 20775 + 831 = 21606
Step 27: 21606 + 831 = 22437
Step 28: 22437 + 831 = 23268
Step 29: 23268 + 831 = 24099
Step 30: 24099 + 831 = 24930
Step 31: 24930 + 831 = 25761
Step 32: 25761 + 831 = 26592
Step 33: 26592 + 831 = 27423
Step 34: 27423 + 831 = 28254
Step 35: 28254 + 831 = 29085
Step 36: 29085 + 831 = 29916
Step 37: 29916 + 831 = 30747
Step 38: 30747 + 831 = 31578
Step 39: 31578 + 831 = 32409
Step 40: 32409 + 831 = 33240

So, 831*40 = 33240