Billionaire investor Warren Buffett got his entrepreneurial start at age 7, buying six-packs of Coke for a quarter, then hawking the beverages for a nickel apiece on hot summer nights. Berkshire Hathaway Inc. agreed yesterday to provide $3 billion of financing for Burger King Worldwide Inc.’s purchase of doughnut chain Tim Hortons Inc. Buffett, Berkshire’s chairman and chief executive officer, has built his career investing in businesses that have broad consumer recognition and appeal. “I’m the kind of guy who likes to bet on sure things,” Buffett said last year at Coca-Cola’s annual meeting.