To find the cost of one ticket to the concert, let's first determine how many people went to the concert and how much they paid specifically for the tickets.
Oksana and her three friends make a total of 4 people. Since there is insurance of $15 for each person, the total cost for insurance is:
\[ \text{Total Insurance Cost} = 4 \text{ people} \times 15 \text{ dollars/person} = 60 \text{ dollars} \]
Now, we subtract the total insurance cost from the total payment to find out how much was spent on the concert tickets:
\[ \text{Total Cost for Tickets} = \text{Total Cost} - \text{Total Insurance Cost} = 540 \text{ dollars} - 60 \text{ dollars} = 480 \text{ dollars} \]
Now, we need to find the cost of one ticket:
\[ \text{Cost of One Ticket} = \frac{\text{Total Cost for Tickets}}{\text{Number of People}} = \frac{480 \text{ dollars}}{4 \text{ people}} = 120 \text{ dollars} \]
Thus, the cost of one ticket to the concert was $120.