Question

an object 0.4 m from a lens gives a virtual image 0.1 m from the lens what is the focal length of this lens what kind of lens is this

Answer 1

$1/a-1/b=1/f\to1/f=1/0.4-1/0.1\to f=-0.133\ (m)$

Since the focal length is negative. It is a diverging lens.