This lesson is optional because it revisits material from earlier grades.

Students begin by revisiting ways to calculate a given percentage of a given number, in preparation for computations they'll need to do in the lesson. Then, they look at a geometric context where three quantities can be related by addition and subtraction. Next, they look at a couple of contexts on spending, earning, and sales tax, which involve multiplication, multiplication and addition, and increasing a number by a percentage.

In each case, students begin by creating models in which the values of the quantities are known (or mostly known), and move toward models in which the quantities are unknown or can change. The repeated reasoning allows students to practice looking for and expressing regularity (MP8). As they interpret verbal descriptions and write equations, students develop their understanding of equations as a way to represent constraints and practice reasoning quantitatively and abstractly (MP2).