The card pinned above the counter is the same one the cook has used at home for years. It serves six people and asks for four hundred and fifty grams of flour, three eggs and half a litre of milk.
Today the hall seats three hundred.
The first thing she writes on the pad is not a quantity. It is a single number: three hundred divided by six is fifty. Everything on the card is now multiplied by that one figure, and nothing else has to be thought about again. The flour becomes twenty-two and a half kilograms. The eggs become a hundred and fifty. The milk becomes twenty-five litres.
The storekeeper asks whether she is sure about the flour, because twenty-two kilograms sounds like a great deal. She checks it the other way round: fifty lots of four hundred and fifty grams, and four hundred and fifty grams is a little under half a kilo, so fifty of them is a little under twenty-five kilos. The two answers agree, and she signs the requisition.
The whole method is in that first line. A word problem is rarely hard in itself; it is hard because the numbers arrive in a jumbled order and in different units. Find the one relationship that governs everything, write it down before anything else, and the rest is multiplication.