Mean Free Path of Magnetic Core
Okay I did the answer:

To find, for example, the mean length of the one at the left:
I started by finding the longest way: started at the middle of the 15 cm, added it all the way to the left then upwards then back to the middle of the 15 cms.

Then I found the shortest way by starting again at the middle of 15 cms then adding until the corner of the square, then upwards to the second corner and then back to the middle of the 15 cms.

I added both "routes" i took and divided by two to obtain the average.