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. Which of these expressions requires a variable?

A
the expression for how much money Abe earns when he makes $15 an hour and works for three hours

B
the expression for the number of pencils Cody has if he starts with nine pencils and then gives four to Molly

C
the expression for the total number of cakes when Cindy brings one cake to class, and Bryan brings two cakes to class

D
the expression for how many birthday parties Pia has this June if she has one more birthday party than her brother