_{I need step by step solution, writting must be clear}

One of the steps in semiconductor processing involves the placement of a stack of silicon wafers into a tube through which a reactant gas is conveyed. The wafers are stacked coaxial with the tube, and the interwafer spacing is 0.5cm. The wafer radius is 7.5cm. When the tube is loaded with a stack of wafers, room air occupies the interwafer space. After the reactor has been loaded, an inert gas is pumped through the reactor. Exchange between the flowing inert gas and the interwafer gas occurs only by diffusion in the interwafer region. The purpose of the inert gas flow is to assist the removal of water vapor from the interwafer space. If the inert gas is dry, how long will it be before the water vapor concentration is reduced to 102 of its initial value? Assume that operation is at atmospheric pressure and 300C. The diffusion coefficient of water in the inert gas is 0.76cm2/s. Assume that there is no adsorbed water on the wafers and that the wafers are impermeable to water