Answer
$$
\mathbf{M}_C=78.1 \mathrm{lb} \cdot \mathrm{ft}
$$
Work Step by Step
$$
\begin{aligned}
\mathbf{r}_{C B} & =\{-3 \mathbf{i}-2.5 \mathbf{j}\} \mathrm{ft} \\
\mathbf{M}_C & =\mathbf{r}_{C B} \times \mathbf{F} \\
& =\left|\begin{array}{ccc}
\mathbf{i} & \mathbf{j} & \mathbf{k} \\
-3 & -2.5 & 0 \\
0 & 0 & 20
\end{array}\right| \\\\
\mathbf{M}_C & =\{-50 \mathbf{i}+60 \mathbf{j}\} \mathrm{lb} \cdot \mathrm{ft} \\
M_C & =\sqrt{(-50)^2+(60)^2}=78.1 \mathrm{lb} \cdot \mathrm{ft}
\end{aligned}
$$
