18. In a children's park amusement area, there are three types of tickets: Ticket A costs 7 yuan, Ticket B costs 4 yuan, and Ticket C costs 2 yuan. One day, the amusement area sold a total of 85 tickets and earned 500 yuan, with Ticket A selling 31 more tickets than Ticket B. How many Ticket A were sold ( ) tickets
56
【Answer】Convert 15.5 tickets of type A to type B, then the revenue becomes 453.5; assuming all 85 tickets are sold as type C, the revenue is 170; and by exchanging two type C tickets for one type A ticket and one type B ticket, an additional income of 7 yuan can be earned; hence $(453.5-170) \div 7=40.5 ; 40.5+15.5=56$ tickets; at this point, 25 tickets of type B are sold, and 4 tickets of type C are sold.