OK, let's crack this. We're given 661 and our task is to multiply it by 25. Essentially, we'll be adding 661 to itself 25 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
Step 16: 9915 + 661 = 10576
Step 17: 10576 + 661 = 11237
Step 18: 11237 + 661 = 11898
Step 19: 11898 + 661 = 12559
Step 20: 12559 + 661 = 13220
Step 21: 13220 + 661 = 13881
Step 22: 13881 + 661 = 14542
Step 23: 14542 + 661 = 15203
Step 24: 15203 + 661 = 15864
Step 25: 15864 + 661 = 16525

So, 661*25 = 16525