What is the probability that a simple random sample will provide a sample of mean within 3 weeks of the population mean?

For unemployed individuals, Barron's reported that the average number of weeks of unemployment in January 2008 was 17.5. Use this value as the population mean and assume that the population standard deviation is 4 weeks. Suppose that a random sample of 50 unemployed individuals will be selected.

What is the probability that a simple random sample will provide a sample mean of at least 20 weeks?