$4-25$ A $90-m^3$ 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 $1\mathrm{.}5Bq*L^{-1}($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 constant of $2\mathrm{.}09{10}^{-6}{s}^{-1}.$ If the source of radon is closed off and the room is vented with

radon$-$free air at a rate of $0\mathrm{.}14m^3*s^{-1},$ how long will it take to lower the radon concentration to an accept$-$

able level of $0\mathrm{.}15Bq*L^{-1}?$