The cost of one ticket will be £8.20
Explanation
Suppose, the cost of one ticket is

As the offer says "buy 3 tickets and get a ticket free" , it means they actually need to pay for 3 tickets.
So, the total cost of 3 tickets

Each of four friends pay £6.15 , so the total amount they pay

Thus, the equation will be .....

So, the cost of one ticket will be £8.20