Entrance tickets to a theme park for an adult cost $14 each. The ticket for a child costs half as much. Last Sunday, 40% of the tickets sold were for children. The total sale of the tickets amounted to $6,720. How many tickets sold were for adults?
If x tickets were sold, then just add up the amounts for adults and children:
0.40x * 14/2 + 0.60x * 14 = 6720