Browse · MATH Print → jmc number theory intermediate Problem Find 9−1(mod100), as a residue modulo 100. (Give an answer between 0 and 99, inclusive.) Solution — click to reveal Note that 9⋅11≡99≡−1(mod100). Then 9⋅(−11)≡−99≡1(mod100), so 9−1≡−11≡89(mod100). Final answer 89 ← Previous problem Next problem →