A vertical cylindrical tank with a flat roof and bottom is to be constructed for storing 150 m³ of ethylene glycol. The cost of material and fabrication for the tank wall is Rs 6000 per m² and the same for the roof and the tank bottom are Rs 2000 and Rs 4000 per m², respectively. The cost of accessories, piping and instruments can be taken as 10% of the cost of the wall. 10% of the volume of the tank needs to be kept free as vapour space above the liquid storage. What is the optimum diameter (in m) for the tank?