The analysis shows that the volume of oil in a rectangular tank is equivalent to two-fifths of that in a cylindrical tank. Divide the volume of oil in the cuboid oil tank by two fifths to get the volume of the cylindrical oil tank, and then divide it by the bottom area to get the height of the oil tank.

The volume of oil in cuboid oil tank =6*3*2=36 cubic decimeter.

The volume of cylindrical oil drum =36/(2/5)=90 cubic decimeter.

Drum height =90/4=22.5 decimeters.