A90-m3 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−1 (becquerels per liter) under steady-state conditions. the room behaves as a cmfr, and the decay of radon is a first-order reaction with a decay rate con- stant of 2.09 × 10−6 s−1 . if the source of radon is closed off and the room is vented with radon-free air at a rate of 0.14 m3 · s−1, how long will it take to lower the radon concentration to an acceptable level of 0.15bq · l−1?