A 24ft high conical water tank has its vertex on the ground and radius of the base is 10 ft. If water flows into the tank at a rate of 20 ft3/min, how fast is the depth of water increasing when the depth of the water is 20 ft?

Answer 1

The vertex is on the ground, so the tank is in a funnel position.
Let the water height be h, then the radius of the surface of water is r(h)=10h/24=5h/12
The volume at a height of h is
V(h)=(π/3)r(h)² h
=(π/3)(5h/12)² h
=(25π/432)h³
Differentiate with respect to time, t

dV(h)/dt
=(25π/432)*3h²dh/dt
=(25π/144)h² dh/dt

Since dV(h)/dt is known (=20 ft³/min), you can solve for dh/dt.

Note that the unit of dh/dt is in ft/min.