"The interview process tests not what the applicant knows, but how well they can process tricky questions: If you wanted to figure out how many times on average you would have to flip the pages of the Manhattan phone book to find a specific name, how would you approach the problem? If a spider fell to the bottom of a 50-foot well, and each day climbed up 3 feet and slipped back 2, how many days would it take the spider to get out of the well? ."

Source: Bill Gates: The Rolling Stone Interview. Interview with Jeff Goodell, www.rollingstone.com. March 13, 2014.