$x^2 + y^2 = 20^2$

$2x \dfrac{dx}{dt} + 2y \dfrac{dy}{dt} = 0$

$x \dfrac{dx}{dt} + y \dfrac{dy}{dt} = 0$

$x \dfrac{dx}{dt} + y (-2) = 0$

$x \dfrac{dx}{dt} - 2y = 0$

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*