You can write 2311 and 3912 in the form :
Then
Taken modulo 20, the terms containing powers of 20 vanish and you're left with
We further have
so we end up with
and so .
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If instead you're trying to find , you can apply Euler's theorem. We can show that using the Euclidean algorithm. Then since , and 8 divides 3912, we have
To show 2311 and 20 are coprime:
2311 = 115*20 + 11
20 = 1*11 + 9
11 = 1*9 + 2
9 = 4*2 + 1 => gcd(2311, 20) = 1