Beth bought 15 tickets to a movie, where adult tickets cost $6.00 and senior citizen tickets cost $4.00. she spent a total of $76. which system of equations will determine the number of adult tickets, a, and the number of senior citizen tickets, s, beth purchased? a + s = 76 6x + 4s = 15 a + s += 15 4a + 6s = 76 a + s = 15 x +4s = 76 a + s = 15 6x + 4s = 76