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 freshmen and 30% are sophomores. if a student is selected at random, the probability he or she is either a junior or a senior is

A student has to be in one of those four ranks.
If a student is not a freshman or sophomore, he must be a junior or a senior.
All students make up 100% of the total number of students.
Since the freshmen are 35% of the students, and the sophomores are 30% of the students, then they make up 65% of all students.
100% - 65% = 35%.
That means the juniors and seniors combined account for 35% of all students.
A student selected randomly has a 35% probability of being a junior or a senior.