A long iron rod initially at a temperature of 20C has one end dipped in boiling water (100C) at time, t = 0. The curved surface of the rod is insulated so that heat conduction is one dimensional in the axial direction. The temperature at a distance 100 mm from the dipped end becomes 40C at time, t = 200 s. The same temperature is achieved at a distance of 200 mm from the dipped end at time ___________________?

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