A narrow, 20 ft (6.10 m) thick vein dips 60◦. Stopping occurs at a depth of 5,100 ft (1,555 m) in rock that averages 165pcf (26.1kN/m3). The vertical stress before mining is equal to the unit weight of overburden times depth; the horizontal premining stress is twice the vertical stress. Unconfined compressive strength is 25,000 psi (172 MPa); tensile strength is 2,850 psi (19.7 MPa). Find the maximum possible extraction ratio assuming no size effect.