A regular room and pillar method is used in a hard rock lead–zinc mine. The ore horizon is at a depth of 1,180 ft where the overburden averages 159pcf. Laboratory tests on core samples (L/D = 2) show that the ore has an unconfined compressive strength of 13,700 psi and a tensile strength of 1,250 psi. 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.