Hi! I have the following questions and I would like some help. I have N different objects and I choose g out of them (without repetition), avoiding permutations of the objects. This can be done in N!/(N-g)!g! ways. I create another set of g objects in the same way. So, I have two such sets of g objects and the question is what is the probability that these two sets have at least one object in common? What is the probability of having exactly one object in common? My actual problem is that i don't count correctly the different ways that I can create two sets with one object i common. Could anyone please help?