Calculating a confidence interval when the sample size is unknown

The problem that I am trying to solve is the following:

...A standardized test is used to assess the students knowledge (national reported mean=65, sd=5). A sample of 30 students are tested (sample mean=58, standard error=3.2). Compute a 99 percent confidence interval based on this sample's data. How do these students compare to the national sample?

I determined the confidence interval of the sample has a lower limit of 49.18 and an upper limit of 66.82. I assume to make the comparison that I need to determine the confidence interval for the information relating to all individuals who have taken the test. After an exhaustive search, I could not find a method of determining the sample size that does not require information that I do not have or that I cannot calculate without the sample size. How can I determine the sample size?