Out of $120$ students, $85%$ found the math test easy. How to determine the $90%$ confidence interval for the population proportion of students who found the math test easy?

I am not sure about my computation but I tried to solve it in the following way.

The $85%$ of 120 students is 102 students.

Since the confidence level is 90%, there will be 92 students

My answer is not an interval but a fixed percentage which is $76.5%$.

Any comments and suggestions will be highly appreciated.