An object is placed at a 40 cm distance from a concave mirror of a focal length of 20cm. The image formed must be: 

Focal length = -20 cm (The focus of a concave mirror is on the left side of the mirror)
Object distance = -40 cm (The object is always placed on the left side of the mirror)

We know that the radius of curvature of a mirror is twice the focal length of a mirror.
$\therefore$ Radius of curvature = 2 x -20 = - 40 cm

$\therefore$ Object distance = Radius of curvature

Since the object is placed at the centre of curvature of a convex mirror, the image formed will be real, inverted and of the same size as the object.