At franklin high school, 15% of students play basketball and 4% play baseball and basketball. what is the probablity that a student plays baseball given that he or she plays basketball?

There is a formula to use for this kind of problem (what is A given B).
P(A | B) = [P (B ∩ A)] /P(B)   which says the probability of A given B equals the probability of B and A over the probability of B.  So if you make baseball A and basketball B you can plug them into the equation:
P(A | B) = 0.04/ 0.15
When you solve I think you should get 27%