Can't figure this one out. I need help Thanks

Sample Size Mean 01a A statistics student would like to estimate the average age of students at her college. She would like to be 95% confident that her estimate includes the actual college population mean. She did a smaller preliminary survey and found that a reasonable estimate of the population standard deviation was 2 1/2 years. How many people must she survey if she wants to be within 1/2 year of the true population mean? Please round OFF your answer to the nearest integrer