She even managed to combine math and acting for one role, in a production of "Proof," the Pulitzer-winning play by David Auburn, in her hometown, San Diego. She played the main character, a young woman who claims to have solved a complicated mathematical proof. "I don't think there is any other time in my life when I knew that this role was supposed to be for me," she said. At an audition, the casting director asked about what she knew of math. Ms. McKellar said she was co-author of a mathematics proof. "She went into a five-minute explanation," said Sam Woodhouse, the artistic director of the San Diego Repertory Theater. "Which was a stunning and mystifying five minutes." Ms. McKellar said she hoped to be a role model for future mathematicians, especially middle school girls. She testified to a Congressional subcommittee in 2000 about how to draw more women into science and math. She has just signed on as spokeswoman for the Math-a-Thon at St. Jude Children's Research Hospital in Memphis, where children work through a book of math problems, and their friends and family pledge money to the hospital for each problem that is solved. For several years, Ms. McKellar has also been answering math questions at danicamckellar.com, under the "mathematics" link. It helps her maintain some of her skills, although she sometimes needs to consult old notes and textbooks. "I have all of them since the seventh grade, except for my ninth-grade geometry book," she said, "which my sister used when she was in ninth grade, and she sold it at the book sale when you sell your books back. "I was like, 'You sold my book?' She's like, 'Yeah.' 'But that was mine.' She's like, 'Oh, oops.' I have every other book." To the person asking about the time it would take to wash a car, Ms. McKellar worked through the calculation of how long it would take if Brian and Sam worked together. The answer: a little less than three and a half minutes. "Yes, I think they should work together," she wrote. "It gets done much more quickly that way."