Students extend their understanding of ratios to develop an understanding of proportions and solve problems, including scale drawings, percent increase and decrease, simple interest, and tax. They also extend their understanding of numbers and properties of operations to include rational numbers. Signed rational numbers are contextualized and students use rational numbers in constructing expressions and solving equations. Students derive formulas and solve two-dimensional area problems, including the area of composite figures. In three dimensions, students find surface area using formulas and nets. Students also compute the volume of three-dimensional objects, including cubes and prisms. Students make use of sampling techniques to draw inferences about a population, including comparative inferences about two populations. Students also investigate chance processes through experimental and theoretical probability models.