I've searched my book and my notes and can't find anything. What formula or formulas do you use to solve this problem? I don't want the answer, I want to know what formula/s to use to solve it myself. I know I have to use the Z table, but I'm not sure how to figure out how and where. I really don't think my teacher went over this in class.

Infinite population has mean of 75, standard deviation of 12. Probability that the mean of a sample of 36 observations, taken at random from this population , is less than 78 is: