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.
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!