What the landlord said should be the second model of the senior high school entrance examination.
I drew a picture.
The black box is the original 18X 18 square paper.
The red frame in the middle is the bottom pattern of the folded cubic carton, and its side length is L.
From the folding process, we can see that the remaining protruding triangle can just fill the cube at the bottom, so the hypotenuse (that is, the red line) of the right triangle is equal to L.
Similarly, when you see the blue line, this section is folded in half twice, just leaving an isosceles right triangle, so the length of the blue line is 2L (because the height is equal to the side length of the bottom).
So we see that L/2+2L+L/2=3L= 18/ (root number 2)
L=3, root number 2.