Question

The solution of the linear congruence 4x = 5(mod 9) is?

(a) 6(mod 9)

(b) 8(mod 9)

(c) 9(mod 9)

(d) 10(mod 9)

The question was asked by my college professor while I was bunking the class.

This intriguing question comes from Applications of Number Theory in chapter Number Theory and Cryptography of Discrete Mathematics

Answer 1

The correct option is (b) 8(mod 9)

Explanation: The inverse of 5 modulo 9 is -2. Multiply by (-2) on both sides in equation 4x = 5(mod 9), it follows that x is congruent to 8(mod 9).