An object, a convex lens of focal length $20 cm$ and a plane mirror are arranged. How far behind the mirror is the second image formed?

Putting $u = - 12 cm$ and $f= + 20 cm$ in the lens formula gives $v = - 30 cm$, i.e., the image is formed at a distance of $30 cm$ from the lens on the same side as the object. The distance of this image. from the mirror  $= 30 + 10 = 40 cm$. Therefore, the second image is formed behind the mirror at a distance of $40 cm$ from it.