There are 10 students in class. Every student sent an email to exactly 5 of them. At least how many people who sent emails to each other?
My approach : consider 10 vertices as 10 students and edges representing the emails
so complete graph with 10 vertices have 45 edges
now changing all undirected edges to directed there will still be no 2 students who are sending emails to each other because we changed undirected to directed so there are no cycles between vertices of length 2.
Now we got 5 remaining edges or emails to send as soon as we'll add these 5 directed edges, we'll get 6 more people at minimum.
Am i going wrong somewhere because answer given is 5.