A regular room and pillar method is used in a hard rock lead–zinc mine. The ore horizon is at a depth of 980 ft where the overburden averages 156pcf. Laboratory tests on core samples (L/D = 2) show that the ore has an unconfined compressive strength of 12,400 psi and a tensile strength of 1,050 psi. Find an algebraic expression for the maximum extraction ratio possible, if no size effect exists, then evaluate the expression for the given data.