Tammy attends tap and ballet classes at her dance studio. Each tap class is 90 minutes long and each ballet class is 60 minutes long. A tap class costs $17 and a ballet class costs $15. Last week, Tammy paid $49 for 240 minutes of dance classes. Which system of equations can be used to determine the number of tap classes, x, and the number of ballet classes, y, that Tammy took last week?