The map in appendix iii shows topographic contour in meters. A planar coal seam with finite thickness is known to exist at site. Four boreholes were drilled across the site. Borehole a intersects the upper and lower boundaries of a coal seam of 900m and 950m respectively. Boreholes b and c encounter the lower boundary at depth of 50m and 150m respectively, For the same thickness of coal seam, draw the upper boundary on the map in appendix