Consider a house into which radon is emitted through cracks in the basement. The total volume of the house is 650 m3 (assume that the volume is well mixed throughout). The radon source emits 250 pCi/s (A picoCurie [pCi] is a unit proportional to the amount of radon gas and indicates the amount of radioactivity of the gas). Air inflow and outflow can be modeled as a flow of clean air into the house of 722 m3/h and an equal air flow out. Radon can be considered to be conservative in this problem. First find the retention time of the house.