7.AT.C.8: Use variables to represent quantities and write and solve equations to model and reason about relationships in context. a. Write equations of the form px + q = r and p(x + q) = r where p, q, and r are specific rational numbers. b. Solve equations of these forms flexibly and efficiently. Explain the solution process and how it connects to the original statement of equality. c. Solve word problems leading to equations of these forms and interpret the solution in terms of the context.