angular magnification

It is the image that is located at infinity, not the object

When the image is at infinity, the object should be located at the focus of the lens. That is, at a distance of f from the lens )f is the focal distance of the lens). Under such situation, the angle subtended by the object at the centre of the lens eand the angle subtended by the image at the centre of the lens are the same.

Hence, the angular magnitude of the image located at infinity
= h/f
where h is height of the object

But when the object is placed at the distance of distinct vision, 25 cm, The angular magnitude of the object
= h/25

Therefore, angular magnification M = (h/f)/(h/25) = 25/f

[notice that you need to compare with the angular magnitude of the object located at distance of distinct vision when calculating the angular magnification].