People put a lot of faith in the anti-typo magic of text books. Of course x can be greater than 15. It can be shown that x must be at least 10, otherwise the rightmost corner of the paper won't even reach the leftmost wall at all. In fact, only when x is greater than 11.848 will L terminate on the right of the page instead of the top of the page ( L(x) is a piecewise function). Perhaps the text accidentally gave away the answer, as the minimum value of L(x) is in fact x=15.
Do you need help with the problem itself or just this setup question?