Total Seat sold = 1200 - 30 = 1170
Assumed all tickets were regular seats:
1170 x 20 = $23 400
Difference in amount collected:
$36 100 - $23 400 = $12 700
Difference in ticket price:
$40 - $20 = $20
Number of premium seats sold:
$12 700 ÷ 20 = 635
Number of regular seat sold:
1170 - 635 = 535
Answer: 535 regular tickets were sold.