Without delay, let's solve this. We've got 855 and we will be multiplying it by 15, that is, adding 855 to itself 15 times.
Step 1: 0 + 855 = 855
Step 2: 855 + 855 = 1710
Step 3: 1710 + 855 = 2565
Step 4: 2565 + 855 = 3420
Step 5: 3420 + 855 = 4275
Step 6: 4275 + 855 = 5130
Step 7: 5130 + 855 = 5985
Step 8: 5985 + 855 = 6840
Step 9: 6840 + 855 = 7695
Step 10: 7695 + 855 = 8550
Step 11: 8550 + 855 = 9405
Step 12: 9405 + 855 = 10260
Step 13: 10260 + 855 = 11115
Step 14: 11115 + 855 = 11970
Step 15: 11970 + 855 = 12825

So, 855*15 = 12825