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38. Rainfall. Statistics from Cornell’s Northeast Regional Climate Center indicate that Ithaca, NY, gets an average of 35.4" of rain each year, with a standard deviation of 4.2". Assume that a Normal model applies.
a) During what percentage of years does Ithaca get more than 40" of rain?
b) Less than how much rain falls in the driest 20% of all years?
c) A Cornell University student is in Ithaca for 4 years. Let represent the mean amount of rain for those 4 years. Describe the sampling distribution model of this sample mean, .
d) What’s the probability that those 4 years average less than 30" of rain?

Having trouble with question "d" not sure what formulas and numbers I need.

If you have c) you pretty much have d).

So your initial population is N(m,std). Assuming independent observations the sample mean for those 4 years has a distribution N( m, std/sqrt(n) ), with n=4. Now all you need to do is find z(30) using that sampling distribution. Then you can find P( xbar < 30 inches)