Suppose $A$ And $B$ Are Events in a Sample Space $S$ , And $P ( A \mid B ) = 1 / 2$

Essay

Suppose $A$ and $B$ are events in a sample space $S$ , and $P ( A \mid B ) = 1 / 2$ and $P ( B ) = 1 / 3$ . What is $P ( A \cap B )$ ?