Sure thing, let's get straight to it. We start with 815 and we're going to multiply it by 21, which means adding 815 to itself 21 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
Step 16: 12225 + 815 = 13040
Step 17: 13040 + 815 = 13855
Step 18: 13855 + 815 = 14670
Step 19: 14670 + 815 = 15485
Step 20: 15485 + 815 = 16300
Step 21: 16300 + 815 = 17115

So, 815*21 = 17115