Tuition and fees for private, nonprofit, four-year colleges have increased to an average of $28,500 for the 2011-2012 academic year, up from an inflation-adjusted $16,276 two decades ago, according to the New York-based College Board, a nonprofit group whose members include universities. For parents, this means a high likelihood that they will need to borrow money. Loans to parents have jumped 75 percent since the 2005-2006 academic year, according to Mark Kantrowitz, publisher of the website FinAid.org. That works out to an average of about $34,000 for those with loans. With interest, the figure rises to about $50,000 over a standard 10-year period — a high level of debt to take on as parents approach retirement and as equity from real estate investments has dropped substantially.