-29. A 90-m basement in a residence is found to be contaminated with radon coming from the ground through the floor drains. The concentration of radon in the room is 1.5 Bq/L under steady-state conditions. The room behaves as a CSTR and the decay of radon is a first-order reaction with a decay rate con- stant of 2.09 10 6 s. If the source of radon is closed off and the room is vented with radon-free air at a rate of 0.14 m/s, how long will it take to lower the radon concentration to an acceptable level of 0.15 Bq/L? -6 S