03 Water flowing into rectangular trough

Volume of water:
$V = 10(3)y = 30y$

$\dfrac{dV}{dt} = 30 \dfrac{dy}{dt}$

$12 = 30 \dfrac{dy}{dt}$

$\dfrac{dy}{dt} = 0.4 \,\, \text{ ft/min}$           answer