OK, let's work through this together. We're starting with 857 and we're going to multiply it by 15, which means we add 857 to itself 15 times.
Step 1: 0 + 857 = 857
Step 2: 857 + 857 = 1714
Step 3: 1714 + 857 = 2571
Step 4: 2571 + 857 = 3428
Step 5: 3428 + 857 = 4285
Step 6: 4285 + 857 = 5142
Step 7: 5142 + 857 = 5999
Step 8: 5999 + 857 = 6856
Step 9: 6856 + 857 = 7713
Step 10: 7713 + 857 = 8570
Step 11: 8570 + 857 = 9427
Step 12: 9427 + 857 = 10284
Step 13: 10284 + 857 = 11141
Step 14: 11141 + 857 = 11998
Step 15: 11998 + 857 = 12855

So, 857*15 = 12855