How to find the distance of an object from the mirror in photography

[WardenOfTheMint]

http://www.picamatic.com/show/2008/08/13/02/806039_901x355.JPG

This is not a 'question.' I'm just trying to understand what trick they pulled to get the 'distance of object from the mirror.' It's probably obvious to many, but not to me! Please help

 

[mgb_phys]

The diagram shows the lens-mirror separation as 25cm.
The questions says that it is midway.