Sampling Distribution Question

I found an question while reading the textbook, but i not really understand how to answer it, hope to get some help from you guys.

A restaurant offers pizza delivery service to a campus dormitory. Delivery times follow a
normal distribution with mean 20 minutes and standard deviation 5 minutes.

1.Suppose the restaurant plans to expand its delivery network, and offers service to
commercial areas as well. A random sample of delivery times is required to estimate the
new population mean of delivery time. How large a sample is needed to ensure the
probability that the sample mean is larger than the new population mean by more than 2
minutes is less than 0.05? Assume the new population standard deviation is 10 minutes.