What is the source for this story about Plato expelling a student?

Question

In "Mathematical Apocrypha Redux", Steven Krantz tells the following tale:

A student of Plato (428 B.C. - 348 B.C.) once asked the great master, "What practical uses do these theorems serve? What is to be gained from them?" Plato's answer was immediate and peremptory. He turned to one of his slaves and said, "Give this young man an obol [a small Greek coin] that he may feel that he has gained something from my teachings. Then expel him."

The story is repeated at King's Way Classical Academy. What is the origin of this story? Is there compelling evidence that it actually happened?

Answer 1

This anecdote is quite common, many versions do not have "expel him", and it is more frequently associated with Euclid rather than Plato. Which is more natural. After all Plato did not teach mathematics, and no mathematical discoveries are credited to him. He discussed mathematics in some of his dialogues, like Theaetetus. "Platonic solids" are named after him, but it was Theaetetus who classified them.

It is not possible to tell whether the story was invented or Euclid or Plato or some other mathematician really said something like this. In fact we know next almost nothing about Euclid, except a couple of anecdotes like this.