A sample of 9 production managers with over 15 years of experience has an average

A sample of 9 production managers with over 15 years of experience has an average salary of $71,000 and a sample standard deviation of $18,000.

Assuming that s = 18,000 is a reasonable estimate for f$sigma f$ what sample size would be needed to ensure that we could estimate the true mean salary of all production managers with more than 15 years experience within $4200 if we wish to be 95% confident?