An absent-minded penn state professor drives from state college to tyrone, 20 miles away, at 60 mph (miles per hour), before realizing that he forgot his wallet. he immediately returns to state college at 60 mph, picks up his wallet and drives back to tyrone at 60 mph. assuming all the travel was done more or less on a straight line and neglecting the time taken to turn around or collect the professor's wallet, what is the magnitude of the professor's average velocity for the entire trip?