A thin biconvex lens is prepared from the glass (μ=1.5) both curved surfaces of which have equal radii of 20 cm each. Left side surface of the lens…

A thin biconvex lens is prepared from the glass $\displaystyle (\mu=1.5)$ both curved surfaces of which have equal radii of $\displaystyle 20$ cm each. Left side surface of the lens is silvered from outside to make it reflecting. To have the position of image and object at the same place, the object should be placed, from the lens at a distance of $\displaystyle \_\_\_\_$ cm.