A plate-and-frame filter press is used to filter a known slurry mixture. At a constant pressure drop of 10 psi, 50 ft3 of filtrate is delivered in 10 min, starting with a clean filter. In a second run with the same slurry and filter press, 40 ft3 of filtrate is obtained in 9 min when the pressure drop is 6 psi, starting with a clean filter. What is the compressibility exponent for the cake if the resistance of the filter medium is negligible?