Answer:
-Wikipedia creates a duplicate website, Wekepedia, with the same content as Wikipedia, but costs $25 a year.
-Wikipedia would remain free to the public.
Explanation:
To solve the problem of free riding the owner of Wikipedia can make a public appeal for support, they can also include paid adverts on Wikipedia pages or even collect little membership fee per year from anyone trying to access their site.
A way of not solving the problem is leaving the site free. Another way of not totally solving the challenge is opening a twin site that collects charge and leaving the main site free: this would make the public to ignore the site involving payment and still stick to the free one.