An object is placed at a distance of 25cm away from a converging mirror of focal length 20cm. Discuss the effect on the nature and position of the image if the position of the object changes from 25cm to 15cm. Justify your answer without using mirror formula .

If the object is placed at 25 cm in front of the concave mirror having focal length 20 cm, this means that the object is placed in between focus and the centre of curvature of the mirror. If the object is placed in front of the center of curvature then the image will be formed beyond the centre of curvature. The nature of the image will be will be inverted. The dimension of the image will be larger than that of object. That is, the image formed is a real image. 

If the object is placed at 15 cm in front of the concave mirror having focal length 20 cm, this means that the object is placed in between focus and the pole of the mirror. An object placed between the pole and focus of a concave mirror forms a virtual and enlarged image because for concave mirror, f is negative and object distance ($u$) is negative but it is given that  $u<f$ therefore image distance will become positive.
This shows that the image formed is virtual and the magnification is $m=-\frac{v}{u}$. As,  $v>0$ that is positive
and $u<0$ that is negative. Thus, enlarged image is formed.