Table 15-4 -Refer to Table 15-4

Table 15-4
$$\begin{array} { | l | l | } \hline \text { Quantity } & \text { Price } \\\hline 10 & \$ 46 \\\hline 20 & \$ 42 \\\hline 30 & \$ 38 \\\hline 40 & \$ 34 \\\hline 50 & \$ 30 \\\hline 60 & \$ 26 \\\hline 70 & \$ 22 \\\hline 80 & \$ 18 \\\hline 90 & \$ 14 \\\hline 100 & \$ 10 \\\hline\end{array}$$
-Refer to Table 15-4. The marginal revenue, when the quantity changes from 30 to 40 units, is