There is not enough information to answer this question without some assumptions.
The general formula for determining the probability of event A happening or event B happening is:
P ( A ∪ B ) = P(A) + P(B) - P ( A ∩ B )
provided that events A and B are independent.
It is reasonable to presume that the probability of a student is male to be 50%. In a predominantly male school, of course, this would be a big mistake.
We have no idea of what the probability of a student making a B is. We also presume that the probability of making a B is not related to the gender of a student. Suppose the probability of getting a B is 20%.
Then the answer to the above question is
0.5 + 0.2 - (0.5)*(0.2) = 0.7 - 0.1 = 0.6, or 60%
NOTE that the probability of event A and event B occurring, provided that they are independent events, is
P ( A ∩ B ) = P(A) * P(B).
