Light – reflection in two facing mirrors

Hi, thanks for stopping by.I’m studying for an exam tomorrow, I would really appreciate even just a push in the right direction!

1. The problem statement, all variables and given/known data
A plane mirror and a convex mirror are placed facing each other and 50 cm apart. A candle is placed on the principal axis 20 cm from the plane mirror, as shown in Figure 17.31. If the distance between the two images in the plane mirror is 40 cm, calculate the focal length of the convex mirror.

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2. Relevant equations
M = Hi/Ho = v/u
1/f = 1/v + 1/u

3. The attempt at a solution
I know I should have attempted a solution, but I read it over and over and I really don’t know where to start! Firstly, what does it mean by "the distance between the two images"? Does the image in the convex mirror act as a second object to the plain mirror? I don’t know what I’m saying…

Attached Images
 Screen Shot 2014-03-23 at 5.55.33 PM.png (10.7 KB)

http://ift.tt/1pnSb0Y