This is represented as $P(X_{1}=1|X_{2}=1)$. According to the definition of conditional probability, this is equal to the joint probability of $X_{1}=1$ and $X_{2}=1$ divided by the probability of $X_{2}=1$. So, we have:
\[P(X_{1}=1|X_{2}=1) = \frac{P(X_{1}=1,
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