In a sample of college students, 40 percent are
third-year students and 70 percent are not
second-year students. What fraction of those
students who are not third-year students are
second-year students?

Lets just say we have 100 students.

So 40 are 3rd year students then 60 are not 3rd year students.

So out of these 60, we want to know how many are 2nd year students.

Looking at the rest of the information, we know that 70 are not second year students, then 30 are second year students.

Therefore 30 out of the 60 are 2nd year students.

30/60 = 1/2