From the figure:

$x = \sqrt{(4t)^2 + 30^2}$

$x = \sqrt{16t^2 + 900}$

$\dfrac{s}{6} = \dfrac{x}{24}$

$s = \frac{1}{4}x$

$s = \frac{1}{4}\sqrt{16t^2 + 900}$

$\dfrac{ds}{dt} = \dfrac{1}{4}\left( \dfrac{32t}{2\sqrt{16t^2 + 900}} \right)$

$\dfrac{ds}{dt} = \dfrac{4t}{\sqrt{16t^2 + 900}}$

when 4t = 40; t = 10 sec

$\dfrac{ds}{dt} = \dfrac{4(10)}{\sqrt{16(10^2) + 900}}$

$\dfrac{ds}{dt} = 0.8 \, \text{ ft/sec}$ *answer*