
$x^2 + y^2 = 20^2$
$2x \dfrac{dx}{dt} + 2y \dfrac{dy}{dt} = 0$
$x \dfrac{dx}{dt} + y \dfrac{dy}{dt} = 0$
$x \dfrac{dx}{dt} + y (-2) = 0$
$x \dfrac{dx}{dt} - 2y = 0$
when x = 16 ft
$16^2 + y^2 = 20^2$
$y = 12 \,\, \text{ ft }$
$16 \dfrac{dx}{dt} - 2(12) = 0$
$\dfrac{dx}{dt} = 1.5 \, \text{ ft/sec}$ answer