Click on the image at right to get the lesson app with instructor notes.In this lesson, students learn that sales tax and tip amounts can be represented in proportion equations of the form y = kx. Students use equations of this form to solve for the tip amount, sales tax amount, and to determine the amount spent given a tip or tax amount. They also identify the constant of proportionality from the equation and from the situation. Students learn that proportion equations can be applied to markups (20% more!) and markdowns (20% off). Students will analyze graphs of these relationships and use these graphs, along with equations, to solve markup and markdown problems. Students will also find the constant of proportionality in the graphs, the equations, and in the situations. Finally, students connect what they have learned about proportion equations to geometric situations—when geometric figures are proportional. Here again, students identify the constant of proportionality and express their results using percents. In this module, students interpret the increase and decrease in each problem.