The manager of a school canteen came back from a fortnight's leave to find that the price list had been taken down for painting and never put back. Nobody could remember what a tea cost.
What she did have were two receipts left in the till drawer. The first was for two teas and three sandwiches, and the total was 31,000 som. The second was smaller: one tea and one sandwich, 12,000 som.
The second receipt is the useful one, and the reason is worth understanding. It tells you that a tea and a sandwich together cost 12,000 — so if you know one price, the other is settled at once.
She used that on the first receipt. Two teas and three sandwiches can be read as two pairs — that is 24,000 som — plus one extra sandwich. The whole bill was 31,000, so the extra sandwich must have cost the difference: 7,000 som. Then a tea is 12,000 minus 7,000, or 5,000.
She checked both receipts before writing the new list. Two teas and three sandwiches: 10,000 plus 21,000 is 31,000 ✓. One of each: 5,000 plus 7,000 is 12,000 ✓.
One receipt on its own would have proved nothing — a tea at 4,000 and a sandwich at 8,000 fits it just as well. It took two independent facts to pin down two unknown prices, and that is not a coincidence about receipts. It is the rule.