The only reason I can see why the probability is zero is that students can only belong to one group...
hehehe..still have problem on this topic.. hope some body can help ..
tq for time
i already have the answer for a & b. i just stuck on c. the ans for c is 0.
With the information you present the only assumption that appears appropriate is that every student belongs to one of the organisations, and that no student belongs to more than one.
With these assumptions the answer for c is zero because no student belongs to two organisations.