Hey I'm trying to get through an assignment for stats. I'm kind of stuck on a question. I know how to calculate mean and standard deviation, my problem is I don't know how to work backwards with these numbers. Which formula would I use to solve the following?

In a national study of educational attainment, you draw a sample of 540 persons and find that the distribution of years of schooling is normally distributed with a mean of 13.5 and a standard deviation of 2.2. With the information given above, please answer the questions as follows:

(a) How many persons in your sample have dropped out of high school, (i.e., have completed less than 12 years of schooling?

I'm really stuck on this so a point in the right direction would help alot.