Probability question....help!

Hi all, new to these forums... looking good so far :)

Basically I have a question I'm having trouble answering (I really don't like this probability stuff! The rest of the subject is okay). I'm sure there will be some other textbook exercises I'll need help with too.

Anyway, here it is:

A manufacturer claims that the net weight of a bag of nails will weigh 400 grams (net). The bags have a normal, standard distribution with a standard deviation of 15 grams.

a) Find the probability that a bag selected at random has a weight of 370 grams or less.

b) Find the probability of 40 bags (selected at random) will have a mean weight of 370 grams or less.

Thanks for your help.