Answer: Hello! An variable is called independent when you have some form of control over it, and is called dependent when ir depends on other variable.
In this problem, you are 5 skateboards, where the difference between them is the brand of wheels, and se how far each skateboard roles without stopping,
Then the dependent variable is how far the skateboard roles without stopping, because this would depend on the wheels in the skateboard, and the independent variable (the thing you are changing) is the wheels in the skateboards.