Let's get this math done. We have 661 and we're going to multiply it by 15. This is the same as taking 661 and adding it to itself 15 times.
Step 1: 0 + 661 = 661
Step 2: 661 + 661 = 1322
Step 3: 1322 + 661 = 1983
Step 4: 1983 + 661 = 2644
Step 5: 2644 + 661 = 3305
Step 6: 3305 + 661 = 3966
Step 7: 3966 + 661 = 4627
Step 8: 4627 + 661 = 5288
Step 9: 5288 + 661 = 5949
Step 10: 5949 + 661 = 6610
Step 11: 6610 + 661 = 7271
Step 12: 7271 + 661 = 7932
Step 13: 7932 + 661 = 8593
Step 14: 8593 + 661 = 9254
Step 15: 9254 + 661 = 9915

So, 661*15 = 9915