Pollutants in auto exhausts.

#1
Hello all - I am in need of some assistance with a probability problem.

The level of nitrogen oxides (NOX) in the exhaust of cars of a particular model varies Normally with mean 0.2 grams per mile (g/mi) and standard deviation 0.05 g/mi. Government regulations call for NOX emissions no higher than 0.3 g/mi.

a. What is the probability that a sing car of this model fails to meet the NOX requirement?

b. A company has 25 cars of this model in its fleet. What is the probability that the average NOX level (x bar) of these cars is above 0.3 g/mi limit?
I am thoroughly confused and I don't know what to do. Any help is greatly appreciated.
 
#2
Ok, I figured out the answer to part A using my calculator. The answer is 0.0228. The only problem I have now is tryignto calculate this with pen and paper.

Also, for part B, would I use this formula to get the answer:

.05 / sqrt(25) = 0.01 ?