Answer
i is the dependent variable; t is the independent variable
Work Step by Step
Given that $i=18-3t$, i is the dependent variable and t is the independent variable.
With this type of equation you are being asked to solve for i based on varying values of t. Because the value substituted for t can be changed arbitrarily, t is called the independent variable. Because the value that is computed for i changes based on the value of t, i is called the dependent variable.
