OK, let's work through this together. We're starting with 666 and we're going to multiply it by 25, which means we add 666 to itself 25 times.
Step 1: 0 + 666 = 666
Step 2: 666 + 666 = 1332
Step 3: 1332 + 666 = 1998
Step 4: 1998 + 666 = 2664
Step 5: 2664 + 666 = 3330
Step 6: 3330 + 666 = 3996
Step 7: 3996 + 666 = 4662
Step 8: 4662 + 666 = 5328
Step 9: 5328 + 666 = 5994
Step 10: 5994 + 666 = 6660
Step 11: 6660 + 666 = 7326
Step 12: 7326 + 666 = 7992
Step 13: 7992 + 666 = 8658
Step 14: 8658 + 666 = 9324
Step 15: 9324 + 666 = 9990
Step 16: 9990 + 666 = 10656
Step 17: 10656 + 666 = 11322
Step 18: 11322 + 666 = 11988
Step 19: 11988 + 666 = 12654
Step 20: 12654 + 666 = 13320
Step 21: 13320 + 666 = 13986
Step 22: 13986 + 666 = 14652
Step 23: 14652 + 666 = 15318
Step 24: 15318 + 666 = 15984
Step 25: 15984 + 666 = 16650

So, 666*25 = 16650