A room containing 1200 cubic feet of air is originally free of carbon monoxide. Beginning at time t = 0, cigarette smoke containing 4% carbon monoxide is introduced into the room at a rate of 0.1 cubic feet per minute, and the well-circulated mixture is allowed to leave at the same rate. Which of these differential equations describes this situation?
a. dy/dt = 0.1 - y
b. dy/dt = .004 - 0.1y
c. dy/dt = .004 + 0.1y
d. dy/dt = .004 - 0.1y/1200
e. dy/dt = .004 -120y
consider things at the end of time interval dt minutes
dy = .04 * 0.1 dt ft^3 of CO has been introduced
That is, dy = 0.004 dt
Now, how much CO has been exhausted during that time?