That was the same guy who went to the garage because his motor was running roughly.
The mechanic got out his screwdriver and turned a screw and the motor ran as smooth as butter. "How much?" asked the man. "Twenty bucks", replied the mechanic. " How do you figure that when you just turned a screw!?", shouted the man. "Yes, I know" said the mechanic," it's 5 cents for turning the screw; it's $19.95 for knowing which screw to turn."