Focal length of concave lens

[Geocentric]

# A convex lens of focal length 10 cm forms a real image of an object placed at a distance of 20 cm from it. Midway between the convex lens and the position of the image a thin concave lens is introduced. The image formed now is at a distance of 5 cm away from the earlier position. What is the focal length of the concave lens?

[sebbysteiny]

I'm sorry,

Optics is by FAR the most boring part of physics. I was sooooo glad when I stopped doing that one. I think that's why nobody's answering.

Is this your homework that you need help with?