Is an independent variable kept constant

Independent variables are variables that change in order to see their effect on another variable. 

Dependent variable is measured quantitavely and affected by the independent variables

In order for the results to be valid, the dependent variable can only be affected by the independent variable, so something needs to be stay constant. The items that need to be kept controlled are actually controlled variables not independent.

The discount is 8.70 . you dont tell us the percent of tax added so there is no way to find out, unless i'm not seeing anything. without tax its 37.70