OK, let's crack this. We're given 813 and our task is to multiply it by 15. Essentially, we'll be adding 813 to itself 15 times.
Step 1: 0 + 813 = 813
Step 2: 813 + 813 = 1626
Step 3: 1626 + 813 = 2439
Step 4: 2439 + 813 = 3252
Step 5: 3252 + 813 = 4065
Step 6: 4065 + 813 = 4878
Step 7: 4878 + 813 = 5691
Step 8: 5691 + 813 = 6504
Step 9: 6504 + 813 = 7317
Step 10: 7317 + 813 = 8130
Step 11: 8130 + 813 = 8943
Step 12: 8943 + 813 = 9756
Step 13: 9756 + 813 = 10569
Step 14: 10569 + 813 = 11382
Step 15: 11382 + 813 = 12195

So, 813*15 = 12195