Two pipes are buried in the earth and maintained at temperatures of 200 and 100C. The diameters

Two pipes are buried in the earth and maintained at temperatures of 200 and 100ºC. The diameters are 9 and 18 cm, and the distance between centers is 40 cm. Calculate the heat-transfer rate per unit length if the thermal conductivity of earth at this location is 1.1 W/m· ºC.