There are 20 wild pigs on an island and the number of pigs double each year for 5 years. The independent variable is

First of all we need to know what are dependent and independent variables.

But lets start with the definition of a variable: Variable is something that we are trying to measure, something whose value depends on other quantities and can change with time.

Dependent variables: It is something that is measured under certain condition or studied under an experiment. For example, how tall a child has grown over past few years.

Independent variables: The variable whose change is not affected by any other variable in the experiment. For example age of something will vary but it is independent of other variables.

Here in our case, time is the independent variable.