The wind chill is the temperature, in degrees Fahrenheit ( ${ } ^ { \circ } \mathrm { F }$ ), a human feels based on the air temperature, in degrees Fahrenheit, and the wind velocity $v$, in miles per hour (mph). If the air temperature is $32 ^ { \circ } \mathrm { F }$, then the wind chill is given by $W ( v ) = 55.6 - 22.1 v ^ { 0.16 }$ and is valid for $5 \leq v \leq 60$. (a) Find $W ^ { \prime } ( 20 )$. Using correct units, explain the meaning of $W ^ { \prime } ( 20 )$ in terms of the wind chill. (b) Find the average rate of change of $W$ over the interval $5 \leq v \leq 60$. Find the value of $v$ at which the instantaneous rate of change of $W$ is equal to the average rate of change of $W$ over the interval $5 \leq v \leq 60$. (c) Over the time interval $0 \leq t \leq 4$ hours, the air temperature is a constant $32 ^ { \circ } \mathrm { F }$. At time $t = 0$, the wind velocity is $v = 20 \mathrm { mph }$. If the wind velocity increases at a constant rate of 5 mph per hour, what is the rate of change of the wind chill with respect to time at $t = 3$ hours? Indicate units of measure.