A regular room and pillar method is used in a hard rock lead–zinc mine. The ore horizon is at a depth of 360 m where the overburden averages 25.2kN/m3. Laboratory tests on core samples (L/D = 2) show that the ore has an unconfined compressive strength of 94.5 MPa and a tensile strength of 8.63 MPa. Find an algebraic expression for the maximum safe extraction ratio, if no size effect exists and a safety factor of 1.75 is required, then evaluate the expression for the given data.