OK, I have your B already and I know how to calculate for every n, but how should the conditional distribution look like? Is it formula which can be derived from this or do I have to "see" what it should look like?
The only thing I can see is that P is between 4 and 11. I cannot come up with a formula that returns this behaviour.
This makes no sense because
1. probability is a number. Your right hand side is not a number.
2. What is the condition X + Y has to satisfy?
And what is meant to mean? Does it mean that Y = 3 or Y = 5?
Plato's replies say exactly what to do. I'm not sure what the trouble can stiil be ....? The result (that is, I assume, the answer) is given in your very first post!