If x is domain and y is range, why am I Mexican?

Ineed to learn how to solve this problem

At the newest animated movie, for every 9 children, there are 4 adults. there are a total of 39 children and adults at the movie.

Mr and mrs smith buy tickets for themselves and their four children. the cost of an adult ticket is ? 6 more than the adult ticket. the total cost of the six tickets is ? 40.50 work out the cost of an adult ticket. in your working let c be the cost of the child ticket and a be the cost of the adult ticket.