A 2.5 m thick coal seam lying at an average depth of 100 m has been developed by bard and pillar method. The width of the square pillars is 30 m (centre to centre) and the gallery width is 4 m. The average density of the overlying strata is 26 kN/m³ and the pillar strength is 4500 kN/m². Extraction ratio during the development of the pillar is