Given the geologic column, strata depths and rock properties in Table 5.8, find:
(a) The maximum possible roof span when mining the lower coal seam full height,
(b) The safe roof span, if a safety factor of 4 with respect to tension is required,
(c) The factor of safety of the massive sandstone (M.S.) roof layer above the sandy shale when the layer below is at maximumspan.