The rate (in cubic feet per hour) that a spherical snowball melts is proportional to the snowball's volume raised to the 2/3 power. (This assumes that the rate snow melts is proportional to the area exposed to the air -- i.e. the surface area of the snowball.) If a snowball of radius 1 foot (volume 4/3 \[Pi] cubic feet) melts in 6 hours, how long will it take a snowball of raduis 3 feet (volume 36 cubic feet) to melt?