OK, let's crack this. We're given 815 and our task is to multiply it by 15. Essentially, we'll be adding 815 to itself 15 times.
Step 1: 0 + 815 = 815
Step 2: 815 + 815 = 1630
Step 3: 1630 + 815 = 2445
Step 4: 2445 + 815 = 3260
Step 5: 3260 + 815 = 4075
Step 6: 4075 + 815 = 4890
Step 7: 4890 + 815 = 5705
Step 8: 5705 + 815 = 6520
Step 9: 6520 + 815 = 7335
Step 10: 7335 + 815 = 8150
Step 11: 8150 + 815 = 8965
Step 12: 8965 + 815 = 9780
Step 13: 9780 + 815 = 10595
Step 14: 10595 + 815 = 11410
Step 15: 11410 + 815 = 12225

So, 815*15 = 12225