Q13/020) Prove that all integer positive powers of 3 have an even tens digit

proof:
we need to show that

3^ n mod 20 is one digit number

n = 1 => 3
n = 2 => 9
n= 3 => 27 = 7 mod 20
n =4 => 21 = 1 mod 20
after n= 4 it repeats and hence 10's digit is even