OK, let's crack this. We're given 915 and our task is to multiply it by 15. Essentially, we'll be adding 915 to itself 15 times.
Step 1: 0 + 915 = 915
Step 2: 915 + 915 = 1830
Step 3: 1830 + 915 = 2745
Step 4: 2745 + 915 = 3660
Step 5: 3660 + 915 = 4575
Step 6: 4575 + 915 = 5490
Step 7: 5490 + 915 = 6405
Step 8: 6405 + 915 = 7320
Step 9: 7320 + 915 = 8235
Step 10: 8235 + 915 = 9150
Step 11: 9150 + 915 = 10065
Step 12: 10065 + 915 = 10980
Step 13: 10980 + 915 = 11895
Step 14: 11895 + 915 = 12810
Step 15: 12810 + 915 = 13725

So, 915*15 = 13725