If the average person breathes 20 times per minute, exhaling each time 100 in³ of air containing 4% carbon dioxide. Find the percentage of carbon dioxide in the air of a 10,000-ft³ closed room 1 hr after a class of 30 students enters. Assume that the air is fresh at the start, that the ventilators admit 1000 ft³ of fresh air per minute, and that the fresh air contains 0.04% carbon dioxide.