Analysis: The teacher informs the first student at 1 minute, and then the teacher and 1 student inform 1 student at the second minute respectively. A *** 1+2=3 students now, and a ***3+4=7 students at the third minute.

Answer: the first minute notice 1 classmate;

You can notify up to 3 students in the second minute;

Up to 7 students can be notified in the third minute;

The fourth minute can inform 15 students at most;

At most 3 1 student can be notified in the fifth minute;

3 1>20.

So it takes at least 5 minutes to notify each player.

Comments: The key to solve this problem is to use the number of students already notified plus the number of students to be notified by the teacher in the next step.