X= denotes number of defective $\begin{array}{|c|c|c|c|c|}
\hline \mathrm{x} & 0 & 1 & 2 & 3 \
\hline \mathrm{P}(\mathrm{x}) & \frac{7}{15} & \frac{5}{12} & \frac{5}{12} & \frac{1}{12} \
\hline \mathrm{x}1{ }^2 & 0 & 1 & 4 & 9 \
\hline \mathrm{P}{\mathrm{i}} \mathrm{x}1{ }^2 & 0 & \frac{5}{12} & \frac{20}{12} & \frac{9}{12} \
\hline \mathrm{p}{\mathrm{i}} \mathrm{x}{\mathrm{i}} & 0 & \frac{5}{12} & \frac{10}{12} & \frac{3}{12} \
\hline
\end{array}\mu=\Sigma \mathrm{p}{\mathrm{i}} \mathrm{x}{\mathrm{i}}=\frac{18}{12}\begin{aligned}
& \Sigma \mathrm{p}{\mathrm{i}} \mathrm{x}1^2=\frac{34}{12} \
& \sigma^2=\Sigma \mathrm{p}{\mathrm{i}} \mathrm{x}_1^2-(\mu)^2
\end{aligned}\begin{aligned} & =\frac{34}{12}-\left(\frac{18}{12}\right)^2=\frac{17}{6}-\frac{9}{4} \ & \frac{34-27}{12}=\frac{7}{12} \ & 96 \sigma^2=96 \times \frac{7}{12}=56\end{aligned}$