What is the remainder when 19^200 is divided by 20?
3
1
-1
2
Correct answer: 1
Solution
19 = (20 - 1). So 19^200 = (20-1)^200. By the binomial theorem, every term except (-1)^200 contains 20 as a factor. (-1)^200 = 1 (even power). Therefore, 19^200 mod 20 = 1.