I just want to confirm I am doing this problem correctly.
The problem asks to compute without a calculator:
3∗25(mod7)
The way I am solving the problem:
3∗25mod
(((3 \bmod 7) * (2 \bmod 7) * (1 \bmod 7)) \bmod 7) \\ (3 * 2 * 1) \bmod 7
6 \bmod 7 = 6
Am I doing this correctly? Just started learning this in class. This is an even number practice problem out the book so I cannot check the answer lol.
Answer
We want to find (3)(2)(x)\pmod{7}, where x is the inverse of 5 modulo 7, that is, where 5x\pmod 7=1.
There are general procedures for finding inverses modulo m, but 7 is a very small number, so we can do it efficiently by trial and error. Note that (5)(3) has remainder 1 on division by 7. So x\pmod 7=3.
Thus we want to compute (3)(2)(3)\pmod 7. This is 4.
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