The U.S. Education Department said it has discovered a calculation error in hundreds of thousands of student financial aid applications sent to colleges this month and will need to reprocess them — a blunder that follows a series of others and threatens further delays to this year’s college applications. A vendor working for the federal government incorrectly calculated a financial aid formula for more than 200,000 students, the department said Friday. The information was sent to colleges to help them prepare financial aid packages but now needs to be recalculated — even as the department works through a backlog of more than 4 million other financial aid applications.