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Last digit of a random power equal to one

Question

Choose mm uniformly from {111,133,155,177,199}\{111,133,155,177,199\} and, independently, nn uniformly from the twenty integers 2004,2005,…,20232004,2005,\dots,2023. Find the probability that the last digit of mnm^n is 11.

Short answer

25\tfrac25. Only last digits matter. Powers of 11 always end in 11 (20 values of nn); 3n3^n and 7n7^n end in 11 when 4∣n4\mid n (5 values each); 9n9^n does when nn is even (10 values); 5n5^n never does. That is 4040 of the 100100 equally likely pairs.

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