Let's get this math done. We have 851 and we're going to multiply it by 15. This is the same as taking 851 and adding it to itself 15 times.
Step 1: 0 + 851 = 851
Step 2: 851 + 851 = 1702
Step 3: 1702 + 851 = 2553
Step 4: 2553 + 851 = 3404
Step 5: 3404 + 851 = 4255
Step 6: 4255 + 851 = 5106
Step 7: 5106 + 851 = 5957
Step 8: 5957 + 851 = 6808
Step 9: 6808 + 851 = 7659
Step 10: 7659 + 851 = 8510
Step 11: 8510 + 851 = 9361
Step 12: 9361 + 851 = 10212
Step 13: 10212 + 851 = 11063
Step 14: 11063 + 851 = 11914
Step 15: 11914 + 851 = 12765

So, 851*15 = 12765