Answer
$
\frac{\partial t}{\partial y} \bigg|_{x=23}
$ degrees in Fahrenheit per degree latitude
Work Step by Step
It is given that $t(x, y)$ is the mean daily temperature in °F at a longitude of x degrees and a latitude of y degrees. So if the longitude is given as 23°, and temperature is changing with respect to latitude, the rate of change is $
\frac{\partial t}{\partial y} \bigg|_{x=23}
$
