Please i really need help. I need the equation and the answer as well. Don't send links they never work.

Suppose a team of pedestrians takes a simple random sample of 100 babies. in this sample, the babies have a head circumference of 46.35 cm and a standard deviation of 1.1 cm. what percent of the time would a random sample of 100 babies a sample mean that is not in the 90% confidence interval? a) 0% b)2.5% c)5% d)10%