In order to set up a rain water harvesting system, a tank to collect rain water is to be dug. The tank should have a square base and a capacity of 250 m3. The cost of land is < 5,000 per square metre and cost of digging increases with depth and for the whole tank, it is< 40,000 h2, where h is the depth of the tank in metres. x is the side of the square base of the tank in metres.