This is a conditional probability, and we can use the definition of conditional probability:
\[
\operatorname{Pr}(A \mid B) = \frac{\operatorname{Pr}(A \cap B)}{\operatorname{Pr}(B)}
\]
where \(A\) is the event \(X \leq 1/2\) and \(B\) is the event \(1/4 \leq X
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