On her Web site, Danica McKellar, the actress best known as Winnie Cooper on the television series "The Wonder Years," takes on questions that require more than a moment's thought to answer. "If it takes Sam six minutes to wash a car by himself," one fan asked recently, "and it takes Brian eight minutes to wash a car by himself, how long will it take them to wash a car together?" "This is a 'rates' problem," Ms. McKellar wrote in reply. "The key is to think about each of their 'car washing rates' and not the 'time' it takes them."...