A20330 cubic foot room initially has a radon level of855 picocuries per cubic foot. A ventilation system is installed that brings in380cubic feet of air per hour that contains 6picocuries 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 of111picocuries per cubic foot.

I am using first order LDE's and I just don't understand what I am doing wrong... could you please show all your work....