# Affine Cipher - Greatest Common Divisor

Really simple question regarding the Affine Cipher.

Can someone tell me how $3^{-1}$ is supposed to result in 9 in the following explanation below?

• 27 ≣ 1 (mod 26) and 3*9 = 27. Gotta pay attention to the modulus. – Chib Nov 6 '16 at 3:42