Three math problems

The table below shows the number of minutes used and the amount charged for some of Sallly's phone calls last month:
Length of calls-----Total cost of calls
12 mins--$0.72
14 mins--$0.56
18 mins--$0.72
21 mins--$0.84
23 mins--$0.92
1. Explain how to find the cost of a 15-minute phone call
2. What is the total cost of a 15-minute phone call? Show all work
3. One month, Sally's total monthly phone bill was $18.56. What is the total number of minutes Sally used her cell phone?