From the Solution 33,

$s = \sqrt{9 - 480t + 11600t^2}$

$\dfrac{ds}{dt} = \dfrac{-480 + 23200t}{2\sqrt{9 - 480t + 11600t^2}}$

$\dfrac{ds}{dt} = \dfrac{-240 + 11600t}{\sqrt{9 - 480t + 11600t^2}} \, \times \, \dfrac{1/t}{1/t}$

$\dfrac{ds}{dt} = \dfrac{-\dfrac{240}{t} + 11600}{\sqrt{\dfrac{9}{t^2} - \dfrac{480}{t} + 11600}}$

after a long time, $t \to \infty$

$\dfrac{ds}{dt} = \dfrac{-\dfrac{240}{\infty} + 11600}{\sqrt{\dfrac{9}{\infty^2} - \dfrac{480}{\infty} + 11600}}$

$\dfrac{ds}{dt} = \dfrac{11600}{\sqrt{11600}}$

$\dfrac{ds}{dt} = \sqrt{11600}$

$\dfrac{ds}{dt} = 107.7 \, \text{ mi/hr}$ *answer*