A 20330 cubic foot room initially has a radon level of 855 picocuries per cubic foot. A ventilation system is installed that brings in 380 cubic feet of air per hour that contains 6 picocuries per cubic foot, while an equal quantity of the well-mixed air in the room leaves the room each hour. Setup and use a differential equation to determine how long it will take for the room to reach a safe to breathe level of 111picocuries per cubic foot.