Task: Question Generation
Input (English): Since 2006, $9 billion has been invested in downtown and surrounding neighborhoods; $5.2 billion of that in has come in 2013 and 2014. Construction activity, particularly rehabilitation of historic downtown buildings, has increased markedly. The number of vacant downtown buildings has dropped from nearly 50 to around 13.[when?] Among the most notable redevelopment projects are the Book Cadillac Hotel and the Fort Shelby Hotel; the David Broderick Tower; and the David Whitney Building. Meanwhile, work is underway or set to begin on the historic, vacant Wurlitzer Building and Strathmore Hotel.[citation needed]
Output (English): How many vacant downtown buildings are there in Detroit?