We have that the total students there are 500. The 12-graders there are 200. Probability is defined as the ratio of positive outcomes of an event, over all the possible outcomes. Suppose we pick student randomly. Then, there are 200 positive outcomes (positive outcome: we pick a student in 12th grade) and there are totally 500 outcomes (we can pick 500 students in total from Riverside High School). This ratio gives: 
. The requested probability is 0.40