Let's get this math done. We have 878 and we're going to multiply it by 15. This is the same as taking 878 and adding it to itself 15 times.
Step 1: 0 + 878 = 878
Step 2: 878 + 878 = 1756
Step 3: 1756 + 878 = 2634
Step 4: 2634 + 878 = 3512
Step 5: 3512 + 878 = 4390
Step 6: 4390 + 878 = 5268
Step 7: 5268 + 878 = 6146
Step 8: 6146 + 878 = 7024
Step 9: 7024 + 878 = 7902
Step 10: 7902 + 878 = 8780
Step 11: 8780 + 878 = 9658
Step 12: 9658 + 878 = 10536
Step 13: 10536 + 878 = 11414
Step 14: 11414 + 878 = 12292
Step 15: 12292 + 878 = 13170

So, 878*15 = 13170