So nine letters were put into three mailboxes, with an average of three letters per mailbox, so there were still 1 letters. If you put them in any mailbox at will, there will always be four letters in one mailbox.

2.7÷6= 1…… 1

Six people come from six different classes, so the remaining 1 people must come from the same class as the previous 1 people, so there must be two people from 1 class.

3. If you want 10 meter, you need100 ÷10+1=1person.

Then the remaining 1 people must stand in the middle of two people in the front 1 1 people, and the distance between them is less than 10 meter.