75. At the bus station, some people have already queued up before ticket sales begin. Now, assuming that after ticket sales start, the number of people added per minute is fixed. If 21 ticket windows are opened, the queue will just disappear after 12 minutes; if 23 ticket windows are opened, the queue will just disappear after 9 minutes. Then, if 27 ticket windows are opened, the queue will just disappear after $\qquad$ minutes.
Reference answer: 6