MathNet 2016-part-01 — 184 problems (EN–VI) Question 120

Students of two groups decided to organize a chess tournament where each student from the first group plays exactly one game with each student from the other group. But one student from the first group and one student from the other one, due to some reasons, failed to participate in the tournament, so the total number of the games in the tournament has been 20% smaller than that of the games planned.
Find all possible numbers of the students participated in the tournament.