Steps
1. setup a table with quantity and value.
2. fill in the table with information from the question.
3. add down to get equations.
4. substittue equations 3 into equation 2.
5. subsititue "y=" into equation one.

tutorial
http://www.onlinemathlearning.com/ticket-problems.html

video
http://pascack.k12.nj.us/cms/lib5/NJ01000238/Centricity/Domain/100/villafrancomurphy.mov