Alexa was coming up to her final exams, which were set across 18 days. Each of the 18 days had 2 sessions, a morning and an afternoon session. Alexa had to complete 6 exams altogether.

When the exam timetable was released, Alexa was very disappointed that she had 2 days on which she had to complete 2 exams, one in the morning and one in the afternoon, on that same day.

Determine the probability that Alexa would find her 6 exams set on just 4 days.