A chemical storage tank is in the shape of an inverted cone with depth 12 meters and top radius 5 meters. When the depth of the chemical in the tank is 1 meter, the level is falling at 0.1 meters per minute. How fast is the volume of chemical changing?

so V= 1/3 pi r^2 h

dh/dt=.1 at h=1 and r=5/12 is what i can gather..

i also got dV/dt= 1/3 pi r^2 dh/dt + 2/3 pi r h dr/dt --- IF i use this equation , how do i find dr/dt

or if i use dV/dt= dV/dh * dh/dt how do i find dV/dh