A variable can be used in an expression that represents a real-world situation where one value is unknown, not stated or can change. Let’s say Jenna is four years older than her sister. If you know that Jenna’s sister is 10, then Jenna is 4 + 10. Jenna is 14. If you don’t know how hold Jenna’s sister is, you can use a variable in an expression to represent Jenna’s age. If we call Jenna’s sister’s age “a,” then Jenna’s age is 4 + a. We could use any letter to represent the variable. 4 + b or 4 + z represent Jenna’s age, too.