1. Let the set $A=\{1,2,3, \cdots, 99\}, B=\{2 x \mid x \in A\}, C=\{x \mid 2 x \in A\}$, then the number of elements in $B \cap C$ is $\qquad$ .
$B=\{2,4,6, \cdots, 98\}, C=\{1,2,3, \cdots, 49\}$, so $B \cap C=\{2,4,6, \cdots, 48\}$. Therefore, the number of elements in $B \cap C$ is 24.