OK, let's crack this. We're given 831 and our task is to multiply it by 18. Essentially, we'll be adding 831 to itself 18 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

So, 831*18 = 14958