By similar triangle:

$\dfrac{10 - y}{30} = \dfrac{4}{x}$

$y = 10 - \dfrac{120}{x}$

$\dfrac{dy}{dt} = - \dfrac{-120\dfrac{dx}{dt}}{x^2}$

$\dfrac{dy}{dt} = \dfrac{120}{x^2}\,\dfrac{dx}{dt}$

when x = 30 - 15 = 15 ft

$\dfrac{dy}{dt} = \dfrac{120}{15^2}\,(5)$

$\dfrac{dy}{dt} = \dfrac{8}{3} \, \text{ ft/sec}$ *answer*