Is it right to condition a RV on dependent RV?

[devonho]

Homework Statement

Let there be two discrete random variables:

<br /> X \in \lbrace 1,2,3,4,5,6,7,8,9,10 \rbrace \quad \text{where } P[X] \text{ is uniformly distributed over the sample space of } X \text{.}<br />

<br /> B = \left\lbrace<br /> \begin{array}{cl}<br /> 1 &amp; \text{if} \quad X&gt;4 \\<br /> 0 &amp; \text{otherwise}\\<br /> \end{array}\right.<br />

<br /> P[B \mid X]=\left\lbrace<br /> \begin{array}{cl}<br /> 0 &amp; \text{if} \quad x \in \lbrace 1,2,3,4 \rbrace\\<br /> 1 &amp; \text{if} \quad x \in \lbrace 5,6,7,8,9,10 \rbrace\\<br /> \end{array}\right.<br />

<br /> P[X] = {1\over10} <br />

<br /> P[X,B] = P[B \mid X]P[X] = \left\lbrace<br /> \begin{array}{cl}<br /> 0 &amp;\text{if} \quad x \in \lbrace 1,2,3,4 \rbrace\\<br /> {1\over 10} &amp; \text{if} \quad x \in \lbrace 5,6,7,8,9,10 \rbrace\\<br /> \end{array}\right.<br />
The above should be agreeable. But what about:

<br /> P[X,B] = P[X \mid B]P<b><br /> </b>

Since B is dependent on X, is it meaningful or even correct to write an expression for P[X|B]?

Homework Equations

The Attempt at a Solution

I think no because the conditional probability will then be recursive.

 

[haruspex]

It's perfectly ok to discuss the probability of an underlying event given an observation concerning it.
P[B=0] = 0.4
P[X=1|B=0] = 1/4
P[X=1 & B=0] = 0.1
etc.