She has a budget of $165, so the total cost of the yoga mat and the classes has to be equal or less than $165.
[tex]C\le165[/tex]The cost is equal to the cost of the yoga mat ($13.25) and the cost of the classes ($12*c, being c the number of classes).
We can write this as:
[tex]C=13.25+12c[/tex]We then can combine both equations and get:
[tex]13.25+12c\le165[/tex]That inequality represents that the total expenses of Natalie have to be equal or less than $165.
Answer: 13.25 + 12c <= 165