finding margin of error for business admin degrees

:?:
Annual starting salaries for college graduates with degrees in business administration are generally expected to be between $30,000 and $45,000. Assume that a 95% confidence interval estimate of the population mean annual starting salary is desired. What is the planning value for the population standard deviation? How large a sample should be taken if the desired margin of error is

a. 500?
b. $200?

I know I have to use formula for sample size for an interval estimate of a population mean. but need help setting this up.

\(\displaystyle \frac{z^{2}s^{2}}{d^{2}}\) Ringing any bells?

You are given d = 500 and d = 200

z^2 you can look up. 1.96?

s^2 is the only tricky one. A good first guess is Range/6.

Note: Don't forget the Finite Population Correction Factor, but it doesn't look like a problem on this one.