To find the cost of one ticket to the concert, we'll start by determining the total amount spent on insurance and then subtract that from the total cost.
- Total cost for tickets: $540
- Number of people: Oksana and her three friends means there are 4 people in total.
- Insurance cost per person: $15
Total insurance cost: \[ \text{Total insurance} = \text{Number of people} \times \text{Insurance cost per person} = 4 \times 15 = 60 \]
Total ticket cost: \[ \text{Total ticket cost} = \text{Total cost} - \text{Total insurance} = 540 - 60 = 480 \]
Cost of one ticket: \[ \text{Cost of one ticket} = \frac{\text{Total ticket cost}}{\text{Number of people}} = \frac{480}{4} = 120 \]
Thus, the cost of one ticket to the concert was $120.