The United States is seventh and falling when ranked world-wide by its proportion of students completing college, as tuition becomes increasingly unaffordable, according to the National Center for Public Policy and Higher Education. The center also gave 43 of the 50 states - including Michigan - a failing grade for their ability to make college affordable for their citizens. Two years ago, 36 states received a failing grade. A major problem is the decline in how much is covered by the Pell Grant, the main federal subsidy to low-income students, the National Center said. The average Pell Grant covered 76 percent of tuition at 4-year colleges and universities in 1990-91; it now covers only 48 percent.