Before a school trip leaves, the coach driver does a piece of arithmetic on a clipboard, and he does it in the same order every time.
The luggage hold under the floor has a maximum load of 900 kilograms. That figure is not the driver's opinion; it is on a plate beside the door, and an overloaded coach handles badly on a mountain road, brakes late and fails its inspection.
Forty-eight pupils are travelling, and each has been told to bring one bag of 15 kilograms. Forty-eight bags at fifteen kilograms is 720 kilograms — the largest single number on his sheet. That leaves 180 kilograms of the limit unused.
Then come the equipment boxes: tents, cooking gear, a first-aid chest, all packed to a standard 20 kilograms each. Divide 180 by 20 and the answer is nine.
Nine, and not a box more. The driver is firm about this even when a teacher points out that a tenth box would only add twenty kilograms to a nine-hundred-kilogram limit. The plate says 900, the sum says 900, and you cannot load part of a box.
What his clipboard shows, in the end, is the shape of every problem of this kind: a fixed ceiling, one large amount already spent, and a whole number of whatever is left.