A room has a volume of 120 m3. An air-conditioning system is to replace the air in

Question:

A room has a volume of 120 m3. An air-conditioning system is to replace the air in this room every twenty minutes, using ducts that have a square cross section. Assuming that air can be treated as an incompressible fluid, find the length of a side of the square if the air speed within the ducts is
(a) 3.0 m/s
(b) 5.0 m/s.