A professor of economics ought to know that if your equation is x/y * y/z, then it doesn't matter at all what Y is. He could use anything he wants for GDP and i

A professor of economics ought to know that if your equation is x/y * y/z, then it doesn't matter at all what Y is. He could use anything he wants for GDP and it won't make a difference in the result. A more interesting point, which I think he was trying to get at, is the idea that US citizens pay around the same taxes per person as other countries, but we produce a much higher GDP per person. We're more productive. Is that good or bad? Is it cause or effect? Discuss.