The mathematical purpose of this lesson is to practice some of the skills that are useful when solving a system of equations. In the associated Algebra 1 lesson, students will solve a system of equations by substituting a number or an expression for a variable, creating an equation in one variable. In the first activity following the Warm-up, students practice solving for one of the two variables in an equation. In the second activity, students practice solving an equation in one variable that will resemble the types of equations they may see after substituting for a variable. 

Students construct viable arguments and critique the reasoning of others (MP3) when they resolve errors by critiquing their partner's work or by explaining their reasoning.