A planoconvex lens has thickness of $4 \mathrm{cm}.$ 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 $3 \mathrm{cm}.$ If the lens is inverted such that the plane face is in contact with the table, the apparent depth of the centre of the plane face is found to be $\frac{25}{8} \mathrm{cm}.$ The focal length of the lens is (assume thickness of lens to be negligible)