A 18000 cubic foot room initially has a radon level of 820 picocuries per cubic foot. A ventilation system is installed that brings in 600 cubic feet of air per hour that contains 3 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 120 picocuries per cubic foot. (Round your answer to 5 decimal places.)