Students at University X must be in one of the class ranks: freshman, sophomore, junior, or senior. At University X, 35% of the students are freshman and 30% are sophomores. If a student is selected at random, the probability that he or she is either a junior or a senior is ______.

A. 35%
B. 65%
C. 30%

Correct Answer: A

Using the addition rule, because freshman and sophomore are disjoint, P(freshmen or sophomore) = P(freshman) + P(sophomore) = 0.35 + 0.3 = 0.65.
The probability of the event junior or senior is 1 minus the probability that this event does not occur, namely 1 minus the probability of the event freshman or sophomore. Its probability is thus 1 - 0.65 = 0.35.