Probability using the Normal Model

This seems like it should be easy because I know the normal model is designated by (N) and I need to find the standard deviation (SD) but I do not think I have the correct formula. If someone could point me in the right direction I think (more like hope) I could get it on my own. Thank you.

The weight of potato chips in a medium-size bag is stated to be 10 ounces. The amount that the packaging machine puts in these bags is believed to have a normal model with a mean 10.2 ounces and a standard deviation of 0.12 ounces. What is the probability that a randomly selected bag is underweight (less than 10 ounces)? Also, some chips are sold in "bargain packs" of 3 bags. What is the distribution of the mean of these three bags?