Beth bought 20 tickets to a movie, where adult tickets cost $8.00 and senior citizen tickets cost $4.00. She spent a total of $140. Which system of equations will determine the number of adult tickets, a, and the number of senior citizen tickets, s, Beth purchased?
a + s = 20
8a + 4s = 140
a + s = 140
8a + 4s = 20
a + s = 20
a + s = 140
a + s = 20
4a + 8s = 140