the end of a car tunnel has the shape of a semicircle on top of a rectangle. the tunnel is exactly 4 km long. calculate the volume of air in the tunnel with no cars in it . the air in a car tunnel must be exchanged frequently. if the exhaust system pumps the air out at a rate of 10m ^ 3 per second, how long does ir take to replace the stale air with fresh air in the entire tunnel ? give your answer in hours and minutes .