Six minutes is enough, the first minute to inform one, the second minute to inform another teacher, the first minute to inform the students to inform the third. By analogy, the total number of people notified per minute is: 1, 3,7, 15. ...

The next one is twice as much as the last one+1, so the number of people in the fifth minute is 3 1, and the number of people in the sixth minute should be 63 >: 40, so the notification is finished in six minutes.

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