Past data shows that on the average 18% of students in a certain class receive an A, 22% a B, 25% a C, and 35% fail. Suppose that in a class of 25 students 3 students get an A, 5 students a B, 6 students a C, and the rest fails. Find the probability of this event (assume independence of the students’ grades). Find also the probability that 11 or more of the 25 students fail.

Help please? i drew a triple venn diagram but i keep coming out with impossible probabilities, like negative numbers.