A concave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification produced is 1/4. The focal length of the lens will be

Concave lens forms the virtual image of a real object.So let u = 4x and v =  - x , then 3x = 10 cmor                   x=103cm∴u=−403cm  and  v=−103cmsubstituting  in  1f=−310+340or       f=−409            or         f=−4.4 cm