Thanks to strong word-of-mouth and weak competition, How to Train Your Dragon flew back to the top of the box office in its fifth week. The week's new releases didn't fare quite so well, with The Back-Up Plan, The Losers and Oceans all disappointing. With no strong newcomer, overall business was down 17 percent from last weekend and 10 percent from the same period last year, when Obsessed topped the box office with $34.8 million. How to Train Your Dragon grossed $18.6 million, rep...