I have some students who have been asking about the mystery letter that they have seen in older students work. They have even seen it come up in their monthly progress monitoring. As third graders, I know they can handle the concepts if it is introduced in a particular way!
These are black and white pages for ease of printing.
These pages never mention the word 'variable', but instead call these problems mysteries. The letter is just a mystery to solve! Each page consists of an introduction that explains the concept, an example of a problem, and then practice questions.
I use this for pull out students as morning work (differentiated than those who are not ready to be challenged) and during RTI classes.
These are also great homework or practice pages for students if this is a concept covered in your core classes!