When an object is placed 10 cm in front of lens A, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 10 cm in front of lens B, the image formed is real, inverted and same size as the object.(a) What is the focal length of lens A?(b) What is the focal length of lens B?(c) What is the nature of lens A?(d) What is the nature of lens B?

(a) The focal length of lens A is 10 cm as only a converging lens forms a real, inverted, magnified image at great distance when the object is placed at the focus $(F)$ of the lens.

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(b) The focal length of lens B is 5 cm as only a converging lens forms a real, inverted, and same size image as the object when the object is placed at the $2F$ position of the lens which is 10 cm $(2\times {5})$. 

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(c) The nature of lens A is converging (convex), as only a converging lens forms a real image.

(d) The nature of lens B is converging (convex), as only a converging lens forms a real image.