A building has a total volume of 20,000 cubic feet. It is specified that the air in the building must be changed every 20 minutes. To meet this requiment, the fan used in this system must have a capacity of ____ CFM.

A building has a total volume of 20,000 cubic feet. It is specified that the air in the building must be changed every 20 minutes. To meet this requiment, the fan used in this system must have a capacity of ____ CFM.