Manuel wants to determine how fast a toy car will move across different surfaces. He lines 3 ramps with different surface covers including wax paper, sandpaper, and smooth carpet and runs the same car down each ramp to determine how long it takes for the car to reach the bottom. What is Manuel's independent variable in this experiment

The independent variable in an experiment is that which you can variate as you wish, to measure the change in other variable (the dependent variable).

So, in this experiment, Manuel's independent variable is the surface, which Manuel change to wax paper, sandpaper and smooth carpet to determine how long it takes the car to run from the top of the rampo to the botton.

Answer: the independent variable is the surface of the ramp.