The ticket machine at the bus station does one thing. You enter how many passengers are travelling, and it gives you a price. Five thousand som for the booking, and twelve thousand for each seat.
Three passengers, then, cost forty-one thousand. The machine works this out in the time it takes the screen to change.
But watch what it refuses. Type nothing and press the green button, and it waits. Type a decimal point and it will not let you — there is no such thing as half a passenger. Try to enter a negative number and the keypad has no way to do it. Type seven and it tells you that the maximum group booking at this machine is six.
So the machine accepts the whole numbers from one to six, and nothing else. That short list is not a limitation the engineers forgot to remove. It is a description of what the question means. A price for two and a half passengers would be an answer to a question nobody asked.
The engineer who wrote the software had to decide each of those refusals deliberately. None of them came from the arithmetic; the arithmetic would happily multiply twelve thousand by minus two. They came from the situation the machine sits in.
The prices it can give are just as fixed: seventeen, twenty-nine, forty-one, fifty-three, sixty-five and seventy-seven thousand. Six possible inputs, six possible outputs, and nothing in between will ever appear on that screen.