A plano - convex lens has thickness 4 cm.When placed on a horizontal table with the curved surface in contact with it,the apparent depth of the bottom-most point of the lens is found to be 3 cm.If the lens is inverted ,such that the plane face is in contact with the table,the apparent depth of the centre of plane face of the lens is found to be `25//8` cm.Find the focal length of the lens.