Give him threepence, since he must make gain out of what he learns.

'threepence' renders τριώβολον "three-obol-piece". This amount increases the sarcasm of Euclid's reply, as it was the standard fee of a Dikastes for attending a court case (μίσθος δικαστικός), thus inverting the roles of teacher and pupil to those of accused and juror. | The English translation is from The History of Greek Mathematics by Thomas Little Heath (1921), p. 357. The quote is recorded by Stobaeus, Florilegium, iv, 114 (ed. Teubner, 1856, p. 205; see also here). Stobaeus attributes the anecdote to Serenus.