A friend of mine gave me the following problem; " You cash you check of x dollars and y cents. You then buy yourself coffee, costing $3.50. However, the waitress gives you the incorrect change of y dollars and x cents." I have attempted to graph the equation but lack the skills to go farther. It was mentioned to me that it could perhaps be done by boolean algrebra, but I have been able to find little more than an encyclopedia entry on this subject.