Answer
v is the dependent variable and t is the independent variable.
Work Step by Step
Given that $v=50-6t$, v is the dependent variable and t is the independent variable.
With this type of equation you are being asked to solved for v 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 v changes based on the value of t, v is called the dependent variable.
