Student tickets for the football game cost $12 each. an adult ticket costs $20. $1720 was collected for the 120 tickets sold at the last game. which system of equations can be used to solve for the number of each kind of ticket sold?
a.x + y = 120; x + 20y = 1720
b.x + y = 120; 12x + 20y = 1720
c.x - y = 120; 12x + 20y = 1720
d.x + y = 1720; 12x - 20y = 120