71. One day, at the entrance of the China Pavilion at the Shanghai World Expo, 945 visitors had already begun waiting to enter. At this time, several people were still coming to the entrance every minute to prepare for entry. In this way, if 4 ticket-checking gates were opened, all waiting visitors could enter in 15 minutes; if 8 ticket-checking gates were opened, all waiting visitors could enter in 7 minutes. Now, it is required that all waiting visitors can enter within 5 minutes, and $\qquad$ ticket-checking gates need to be opened.
Reference answer: 11