According to a brochure from Dow Chemical Company, the pressure drop for water flowing through a bed of 20-mesh to 50-mesh Dowex 50-X8 resin (spherical beads: 20-mesh diameter = 0.833mm, 50-mesh diameter = 0.287mm) is said to be proportional to the flow rate and has a value of 0.80 psi/ft at a flow rate of 10gpm/ft2. Assuming an arithmetic average particle size, what void fraction would be needed to agree with the published pressure drop?