The rule of the average is
[tex]A=\frac{f(b)-f(a)}{b-a}[/tex]
Since we need to find the average speed in the interval 1 to 3 seconds, then
f(1) = 10.5 ------- a = 1
f(3) = 94.5 ------ b = 3
Substitute them in the rule above
[tex]\begin{gathered} A=\frac{94.5-10.5}{3-1} \\ A=\frac{84}{2} \\ A=42 \end{gathered}[/tex]
The average speed is 42 m/s
The correct option is D
