(a) Explain the different modes of heat transfer with appropriate expressions. (b) A composite wall consists. of 10 cm thick layer of building brick, k = 0.7 W/mK and 3 cm thick plaster, k 0.5 W/mK. An insulating material of k = 0.08 W/ mK is to be added to reduce the heat transfer through the wall by 40 %. Find its thickness.