Remember how Barbie and Obama agreed that math is hard? Now we know why President Barack Obama couldn't recall the national debt during a chat with David Letterman a few weeks back. He's got something in common with a certain iconic doll - Math is hard. Obama appeared on "The Tonight Show" last night, and he got into a soft and fuzzy exchange with host Jay Leno about his daughter's homework. "The math stuff I was fine with up until about seventh grade ... Malia's now a freshman in high school, and I'm pretty lost," Obama confessed to Leno. [youtube]http://www.youtube.com/watch?v=cEEpVXspflM[/youtube]...