I think that this is your question
Enrollment at a local university is 8,500. There are 1/3 students over 25. How many students are 25 and under and how many are over 25?
If 1/3 is over 25 then 2/3 must be 25 or more.
2/3 of 8500 = 5666.6
Lets round that to 5667 students