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When working modulo m, the notation a^-1 is used to denote the residue b for which ab = 1 (mod m), if any exists. For how many integers a satisfying 0 < a < 100 is it true that a*(a - 1)^{-1} = 10a^{-1} (mod 20)?

 Dec 24, 2020
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Hi! It's a pretty long process, so I won't post how i got the answer, but I got 7. Hope it's correct!!

 Dec 24, 2020

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