TOT 113 1986 Spring J7 30 pupils from the same class exchange visits
Source:
August 29, 2019
combinatorics
Problem Statement
Thirty pupils from the same class decided to exchange visits. Any pupil may make several visits during one evening, but must stay home if he is receiving guests that evening. Prove that in order that each pupil visit each of his classmates
(a) four evenings are not enough
(b) five evenings are not enough
(c) ten evenings are enough
(d) even seven evenings are enough