I don't understand why in this example problem the textbook authors chose the values they did for the domain of the the variable. Any clarification would be welcome!

The lower right-hand corner of a piece of paper is folded over to reach the leftmost edge, as shown. If the page is 20 cm wide and 30 cm long, what is the length L of the shortest possible crease?

The text states that $\displaystyle 0 \leq x \leq 15$. I don't understand why it is not $\displaystyle 0 \leq x \leq 20$ -- that is what I would have used.

How to you know just looking at the basic problem information that the maximum possible value for x is 15?