A thin convelx lens L (refective index =1.5) is placed on a plane mirror M. when a pin is placed at A, such that OA=18 cm, its real inverted image is formed at A itself, as shown in figure. when a liquid of refractive index `mu_(i)` is put between the lens and the mirror, the pin has to be removed to A',. such that OA'=27 cm, to get its inverted real image at A' itself. the value of `mu_(i)` will be
