Exposure to air containing more than 0.012 percent carbon monoxide (by volume) is considered dangerous. A 1000 cubic foot room is initially free of carbon monoxide. Beginning at time t = 0, cigarette smoke containing 4 percent carbon monoxide (by volume) is blown into the room at a rate of 0.1 cubic feet per minute. The well-circulated air leaves the room at the same rate. How long does it take for the level of carbon monoxide in the room to reach a dangerous level?