Beginning Algebra With Applications, Chapter 5, 5.1, Section 5.1, Problem 38

The table shows the temperature, in degrees Fahrenheit, of a cup of coffee, measured every 10 min over a period of one and one-half hours.

$\begin{array}{c|cccccccccc}
\text{Minutes} & 0 & 10 & 20 & 30 & 40 & 50 & 60 & 70 & 80 & 90 \\
\hline\\
\text{Temperature, $^{\circ} F$} & 177.3 & 144.6 & 122.3 & 107.0 & 96.6 & 89.4 & 84.6 & 81.2 & 78.9 & 77.4
\end{array} $

During which 30-minute interval was the average rate of change closest to 1 degree per minute?

Based from the table, the average rate of change closest to -1 degree per minute from a 30-minute interval is from 10 minutes to 40 minutes, which is


$
\begin{equation}
\begin{aligned}

\text{average rate of change} =& \frac{\text{temperature in 40 minutes - temperature in 10 minutes}}{40-10}
\\
\\
=& \frac{96.6-144.6}{40-10}
\\
\\
=& -1.6^{\circ}

\end{aligned}
\end{equation}
$