Madina owns a small patisserie in Samarkand. Over the years she has noticed a steady pattern: when she charges ten dollars for a cake, she sells thirty cakes a week, and for every dollar she adds to the price she sells one cake fewer.
She hired a consultant to review last month's records, which a new cashier had kept badly. The consultant's report had one line about cakes: weekly revenue from cakes was 300 dollars. Madina wanted to know what price the cashier had actually charged.
The answer was not as simple as it looked. At ten dollars she sells thirty cakes, which brings in 300 dollars. But at thirty dollars she sells only ten cakes, and that also brings in 300 dollars. There are two possible prices. The report's line, true as it was, could not tell her which one had been used.
Madina's daughter, who was preparing for the GMAT, recognised the situation at once. "In a value question," she said, "information is sufficient only if it gives exactly one value. Two prices means not sufficient, even if both are correct."
They looked for a second fact. The cashier's shift notes showed that no cake had been priced above twenty dollars that month. That single line rules out the higher price. Together with the report, it left only one answer: ten dollars.
Madina added a column for prices to her spreadsheet the same day. A total, she realised, hides the details that produced it, and the same total can come from very different decisions.