Calculating Adult Tickets Sold

At a school event, there are two types of tickets being sold: adult tickets and child tickets. The price of an adult ticket is $15, and the price of a child ticket is $10. If the total revenue from selling 50 tickets was $650, how many adult tickets were sold?

Let the number of adult tickets sold be represented by the variable $a$, and the number of child tickets sold be represented by the variable $c$. The system of equations formed from the problem is:

1. $a + c = 50$

2. $15a + 10c = 650$

Use these equations to determine the number of adult tickets sold.