From the figure:

$\dfrac{x}{y} = \dfrac{x + 15}{20}$

$20x = xy + 15y$

$(20 - y)x = 15y$

$x = \dfrac{15y}{20 - y}$

$\dfrac{dx}{dt} = \dfrac{(20 - y)\left(15\dfrac{dy}{dt}\right) - 15y\left( -\dfrac{dy}{dt} \right)}{(20 - y)^2}$

$\dfrac{dx}{dt} = \dfrac{15(20 - y) + 15y}{(20 - y)^2}\,\dfrac{dy}{dt}$

$\dfrac{dx}{dt} = \dfrac{300}{(20 - y)^2}\,\dfrac{dy}{dt}$

when y = 5 ft

$\dfrac{dx}{dt} = \dfrac{300}{(20 - 5)^2}\,(6)$

$\dfrac{dx}{dt} = 8 \, \text{ ft/sec}$ *answer*