From New Scientist #1364, 30th June 1983 [link]
The professor during his lecture on relativity asked: “If I am in a spacecraft travelling at half the speed of light and pass another craft travelling in the opposite direction at a quarter of the speed of light, what is our relative velocity?”
“Three quarters of the speed of light,” replied one student.
“You weren’t paying attention at my last lecture,” said the professor. “We proved that, according to the special theory of relativity, when two velocities are to be added then the result is not their sum but this,” he broke off to write on the board then continued, “where c is the velocity of light — 300 000 kilometres per second.
“Is it possible for two equal rational velocities to be added so that the result is an integral number of 1000 kilometres (we shall say megametres) per second?”
“No professor,” answered a bright student. “But if the speed of light is decreased by an integral number of megametres per second then it is possible.”
“But you can’t reduce the speed of light! — It is constant,” protested the professor.
“But we can imagine it to be less,” persisted the student.
The professor then suggested the amount his student had taken as the velocity of light.
“I took it as more than that, professor.”
“In that case I calculate what you took as the speed of light and all the possible sums of the equal velocities.”
Note: I am waiting for a phone line to be connected at my new house, so I only have sporadic access to the internet at the moment.