In one day at a library, 64 people borrowed books. Twice as many people borrowed only a thriller as borrowed only a science fiction book. Three people borrowed a biography only and 11 people borrowed both science fiction and a thriller, but not a biography. The same number borrowed a biography and a thriller but no science fiction as borrowed one of each of the three types. Twenty-one people did not borrow a thriller. One more person borrowed a science fiction book and a biography book than borrowed a biography only.　　How many people borrowed a thriller only?