A thin convex lens L (refractive 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 ${\mu }_1$  is put between the lens and the mirror, the pin has to be moved to A’, such that OA’ = 27 cm, to get its inverted real image at A’ itself. The value of ${\mu }_1$  will be