The BSU Track Team is planning to sell t-shirts at the NSIC Conference Track Meet. Thunderbird Graphics charges $40 to set up a screen print with the NSIC Logo ($40 is called a fixed cost. No matter how many t-shirts they purchase they still need to pay $40.) and $5.25 a t-shirt (this cost depends on how many t-shirts they sell, so it is a variable cost.). They plan to sell the t-shirts for $19 a shirt.

Consider the linear equation that gives the amount spent in terms of the number of t-shirts purchased.

What should the independent variable represent?