Answer to Parsimonious Professor

The professor filled the 29 fl oz glass and measured out 10 fl oz. 19 fl oz remained. The glass was cylindrical, so the Professor could now tilt it as shown to measure out 11 fl oz (the amount halfway between the 7 and 29 fl oz marks). Of course, he had to discard one extra fl oz, or return it to stock. 10+11 gave him 21 fl oz.