Explanation:
1) if total number of the saled tickets is 189, then it is possible to write s+a=189, where s - number of student tickets, a - adult tickets.
2) if the ratio is 4:5, then it is possible to write s:a=4:5;
3) using the two equations it is possible to make up the system:
4) Part A: after solution the system above the number of student tickets = 84; adult tickets = 105.
5) Part B: the revenue from selling the adult tickets is: 105*7=735, the student tickets is: 84*5=420. Then the required difference is: 735-420=$ 315.