Let $R$ be a commutative ring with unity 1 which is not a field. Let $I\subset R$ be a proper ideal such that every element of $R$ not in $I$ is invertible in $R$. Then find the number of maximal ideals of $R$?
Now since $I$ does not contain any invertible element so $1$ does not belong to $I$, so $I\not=R$. But how to count the number of maximal ideals? I want hint to solve this problem. Can anyone provide me a hint please? Thank you.