A room and pillar trona mine at a depth of 1,560 ft (475 m) develops pillars 30 ft (9.14 m) wide and 60 ft (18.3 m) long in a flat seam 12 ft (3.66 m) thick. Entries and crosscuts are 24 ft (7.32 m) wide. Laboratory tests of trona core 2-1/8 in. (5.4 cm) in diameter and 4-1/4 in. (10.8 cm) long give an average unconfined compressive strength of 6,740 psi (46.5 MPa) and a coefficient of variation of 38%. A size effect is indicated. If a pillar safety factor with respect to compressive failure of 1.67 is required, estimate the maximum allowable extraction ratio?