Computing Average Rate of Change from a Table

After picking up a friend who lives 10 miles away and leaving on a trip, Anna records her distance from home over time. The values are shown in the table. Find her average speed over the first 6 hours.

$$\begin{array}{ccccccccc}\text{t (hours)}& \text{0}& \text{1}& \text{2}& \text{3}& \text{4}& \text{5}& \text{6}& \text{7}\\ \text{D(t) (miles)}& \text{10}& \text{55}& \text{90}& \text{153}& \text{214}& \text{240}& \text{292}& \text{300}\end{array}$$

OpenStax, Algebra and Trigonometry 2e · §3.3 Rates of Change and Behavior of Graphs · Example 2