7. A local foundation organized a lunch party for all the kids in the
community. They ordered a total of 500 burger meals. They ordered plain
burgers at $1.75 each, cheeseburgers at $2.50 each, and chicken burgers
at $3.20 each. They ordered mostly cheeseburgers, and 100 fewer
chicken burgers than plain burgers. The total order came to $1,172.50.
Define the variables and write a system of equations to represent the
situation.