The PDF linked below challenges stuents to use knowledge of algebraic notation to write expressions, including how to quantify the distributive property, and combine the expressions into equations for a context situation about ages. A student must be able to solve three equations with two variables. A student must be able to evaluate the meaning, utility, and reasonableness of results of symbolic manipulations. A student must be able to use the knowledge of equality to change an equation based on elapsed time.
http://www.insidemathematics.org/common-core-math-tasks/high-school/HS-F-2007%20How%20Old%20Are%20They.pdf