The Wind-Chill Index $I$ Is the Perceived Temperature When the Actual Temperature Is

The wind-chill index $I$ is the perceived temperature when the actual temperature is $T$ and the wind speed is $v$ so we can write $I = f ( T , v )$ . The following table of values is an excerpt from a table compiled by the National Atmospheric and Oceanic Administration. Use the table to find a linear approximation $L ( T , v )$ to the wind chill index function when $T$ is near $16 ^ { \circ } \mathrm { C }$ and $v$ is near $30 \mathrm { kmh }$ .
$\begin{array} { | l | c | c | c | c | c | } \hline T & 10 & 20 & 30 & 40 & 50 \\\downarrow & & & & & \\v & & & & & \\\rightarrow & & & & & \\\hline 20 & 18 & 17 & 14 & 13 & 12 \\\hline 16 & 15 & 12 & 10 & 7 & 8 \\\hline 12 & 8 & 6 & 2 & 2 & - 1 \\\hline 8 & 5 & 1 & - 2 & - 6 & - 7 \\\hline\end{array}$