Focal length of the plano-convex lens is
`(1)/(f)=((3)/(2)-1)((1)/(10)-(1)/(oo))`
or `f=20cm`
If point object Ois placed at a distance os 20 cm from the plano-convex lens, rays becme parallel and final image is formed at second focus or 20 cm from concave lens which is independent of b.