The mean incubation time of fertilized chicken eggs kept at 100.5F in a still-air incubator is 21

Question:

The mean incubation time of fertilized chicken eggs kept at 100.5°F in a still-air incubator is 21 days. Suppose that the incubation times are approximately normally distributed with a standard deviation of 1 day.
(a) Determine the 17th percentile for incubation times of fertilized chicken eggs.
(b) Determine the incubation times that make up the middle 95% of fertilized chicken eggs.