The mean of incubation time of fertilized chicken eggs kept

The mean of incubation time of fertilized chicken eggs kept at 100.5 F in still-air incubator is 21 days. Suppose that the incubation times are normally distribuated with a standard deviation of 1day.a) What is the probability the mean incubation time will last 20 days ? b)Determine the incubation times that make up the middle 95% of fertilized chicken ?