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Tickets bitte.

The Düsseldorf tram system has vending machines to sell tickets. The ticket is rectangular, costs €2.30, and is good for an hour. In order to validate the ticket you need to shove it into a slot of a special stamping machine before you set out for your journey. The machine senses the ticket, and stamps the date & time on its edge. There is also a discount ticket for 4 rides, which costs €8.40. Now the question - why does this system limit you to 4 rides, and not, say, 5 or 8?