The ratio of tenth graders to the school's total population is 86:255 = 33.7%.
The ratio of eleventh graders to the school's total population is 18:51 = 35.3%.
Since the probability of a student being in either tenth, eleventh, or twelfth grade = 1 = 100% (that is, certainty), then the probability of a randomly drawn student being in twelfth grade is (100-33.7-35.3)% = 31.0%.
When randomly choosing one student from the whole school, it is most likely (35.3%) that the student is in the eleventh grade.
