The teacher sent out notices one by one, and it took 15 minutes to finish.

Option 2:

The teacher informed eight students, and everyone informed the others 1 person.

8+ 1=9 minutes.

Scheme 3: The teacher informs all the notified people to keep informing others.

The first minute: the teacher informed 1 person.

The second minute: just now, the teacher and classmates informed two people.

The third minute: the teacher and the students inform four people together.

Fourth minute: The teacher and classmates will notify 8 people together.

1+2+4+8= 15

It will be notified in four minutes.

Many hands make light work = = ... The more people are notified, the faster they will be.