Question 1168085
<pre>

The trick is to open the box out flat.  Then it's easy as can be:

{{{drawing(400,2800/9,-1,17,-1,13,
locate(1.8,12.7,A), locate(14,0,B),red(line(2,12,14,0)),
line(0,2,2,2),line(2,2,2,0),line(2,0,14,0),line(14,0,14,2),
line(14,2,16,2),line(16,2,16,10),line(16,10,14,10),line(14,10,14,12),
line(14,12,2,12),line(2,12,2,10),line(2,10,0,10),line(0,10,0,2),
green(line(2,10,2,2),line(2,2,14,2),line(14,2,14,10),line(14,10,2,10))
)}}}

Just find the length of the red line using the Pythagorean
theorem. The red line is the hypotenuse of an isosceles right 
triangle.

Edwin</pre>