Google posted the billboard above around Silicon Valley as part of their recruiting campaign:
the billboard (which offers problem-solvers the URL to, sorry, a page containing an even harder problem), is a recruiting campaign. We’ve always worked hard to hire the smartest engineers we can find, and we thought this would be a cool way to find a few more.
The answer to the question above is 7427466391.
Update: An interesting, long discussion about the solutions is available.
