This video looks at several different problems for evaluating expressions with multiple variables. The first couple problems are simpler, though the last few focus more on fractions and decimals.
Khan Academy exercises to practice:
1. Evaluating Expressions with Multiple Variables: https://www.khanacademy.org/math/alge...
2. Evaluating Expressions with Multiple Variables: Fractions and Decimals: https://www.khanacademy.org/math/alge...
EulersAcademy.org