Answer
The outcomes in event A are:
\[A=\left\{ \begin{align}
& SSBBBB,\text{ }SBSBBB,\text{ }SBBSBB,\text{ }SBBBSB,\text{ }BSSBBB, \\
& BSBSBB,BSBBSB,BBSSBB,BBSBSB,BBBSSB \\
\end{align} \right\}\]
Work Step by Step
Event A represents that a batter walks on the sixth pitch. A batter “walks” if the fourth ball is called before the third strike. The umpire can call each pitch either a ball (B) or a strike (S). Hence, the outcomes of event A will contain only 2 strikes (S), and the last pitch is a ball (B).
The outcomes of event A are:
\[A=\left\{ \begin{align}
& SSBBBB,\text{ }SBSBBB,\text{ }SBBSBB,\text{ }SBBBSB,\text{ }BSSBBB, \\
& BSBSBB,BSBBSB,BBSSBB,BBSBSB,BBBSSB \\
\end{align} \right\}\]
