3 economists and 3 mathematicians were going to an academic conference by train. Before the journey, the mathematicians bought 3 tickets but economists only bought 1. The mathematicians were glad their stupid colleagues make such an error which verified their view that economists can’t perform even simple mathematics and were going to pay a fine. However, when the conductor was approaching their compartment, all 3 economists went to the nearest restroom. The conductor, noticing that somebody was in the toilet, knocked on the door. In reply he saw a hand with 1 ticket. He checked it and the economists saved 2/3 of the ticket price.
The next day, the mathematicians decided to use the same strategy. So they bought only 1 ticket, but, this time, the economists did not buy any tickets at all.
When the mathematicians saw the conductor, they hid in the toilet, and when they heard knocking they handed in the ticket. They did not get it back.
The economists took it and went to the other toilet.