A 400-m, one-way, two-lane rectangular tunnel (9 x 4 m) filled with cars is blocked by an accident at the exit. The ventilation system involves large exhaust fans blowing ambient air out of the exit end of the tunnel. Assume that the tunnel is full of cars, all with engines idling. One car occupies 6 linear meters in one lane and emits 2.0 g/min CO. The fans pull 0.42 x 106 m3/min of fresh air containing 0.5 mg/m3 Co into the tunnel entrance. What is the concentration of CO in the exit air? Assume complete mixing.