3A One class in Grade 5 of Olympic Mathematics 13 students. Please explain that there must be two students in the same constellation among the students in 13.

There are twelve people in the constellation 1 * * *, but there are thirteen people, assuming that the constellations of the first twelve people are not repeated. Then the thirteenth person must repeat one of them.

This kind of problem is not particularly difficult. It can be described in words: there are twelve constellations, but there are thirteen people.

So 13- 12= 1.

So it must be two people in the same constellation.