@visceral,
P=(probability of something)
A=students that passed first test
B=students that passed both tests
So, using conditional probability,
P(A and B) = P(A)*P(B/A)
Divide both sides by P(A)
P(A and B)/P(A)=P(A)*P(B/A)/P(A)
Cancel **** out
P(A and B)/P(A)=P(B/A)
Commute that ****
P(B/A)=P(A and B)/P(A)
So, figure that **** out... 60%