Each ticket is €8.20,
They each payed €6.15 and there were four friends.
6.15 x 4 = 24.60
€24.60 was the total they payed in total but now you have to divide it by 3 since it was buy 3 tickets get one free
one you divide €24.60/3= €8.20
each ticket was €8.20
hope this helps!