Note: For obfuscated graph questions, remember to explain how the problem translates into graph terms.

Rohan noticed that a large number of Penn students went to Europe for spring break. Consider a graph where the vertices are Penn students in Europe and 2 vertices are linked by an edge if those 2 students are in the same city at the same time. Assume each Penn student has 9 other Penn students in the same city at the same time. Prove that in that case, this graph cannot have 42 edges.