If a researcher's hypothesis is that the greater amount of time teenagers spend playing video games, the higher their grades in math, then the independent variable is.

When doing research, we usually come across two different types of variables. One is the independent variable and the other one is the dependant variable.

The independent variable is the one that we, as researchers, can control and that has an effect on another variable.
The dependant variable is the variable that we can't control and that is affected by the independent variable.

Therefore, we can control the independent variable but not the dependant variable. And the effect we will be observing will take place in the dependent variable.

In this example, the hypothesis is that the greater amount of time teenagers spend playing video games, the higher their grades in math. We can see that the hypothesis states that the math grades will be AFFECTED by the amount of hours teenagers spend playing video games. In other words, the amount of time spent playing will have an effect on the grades. Therefore, the independent variable is The amount of time spent playing video games.