Volume of water:

$V = \frac{1}{2}xy (10) = 5xy$

By similar triangle:

$\dfrac{x}{y} = \dfrac{4}{4}$

$x = y$

$V = 5y^2$

$\dfrac{dV}{dt} = 10y \dfrac{dy}{dt}$

when y = 6 in or 0.5 ft

$3 = 10(0.5) \dfrac{dy}{dt}$

$\dfrac{dy}{dt} = 0.6 \, \text{ ft/min}$ *answer*