Kevin Carey, director of the education policy program at New America, writes:  Every spring, colleges send two sets of letters to millions of prospective students. First the good news: an acceptance letter. Then, a few weeks later, a “financial-aid award letter,” theoretically explaining how much college will cost.  Parents and students are supposed to use this second letter to make one of the biggest financial decisions of their lives. But the task is often nearly impossible, as shown by an analysis from my colleagues at the think tank New America, working with uAspire, a nonprofit that provides financial-aid counseling.