Rustam manages a logistics company in Tashkent. One Monday the board asked him a simple question: how many trucks did the company use last month?
His analyst, Sevara, came back with two facts. The first was the total fuel bill for the month: 36,000 dollars. The second was the fuel cost of a single truck for the month: 1,800 dollars, the same for every truck.
Rustam did not reach for a calculator. He asked a different question first: is that enough to answer the question? The total bill alone was not enough, because the same bill could come from twenty trucks or from forty trucks that drove less. The cost of one truck alone was not enough either, because it said nothing about how many trucks there were. Neither fact answered the question on its own.
Together, however, the two facts were enough. If the whole fleet cost 36,000 dollars and each truck cost 1,800, the company had used 20 trucks. Only then did Rustam do the division.
A week later the board asked a second question: what was the average fuel cost per truck? This time Rustam answered at once. Sevara's second fact already said that every truck cost 1,800 dollars, so the average was 1,800. The total bill was not needed at all.
Rustam explained his habit to a new trainee. Collecting data costs time and money, he said, and in a busy week nobody wants a report that cannot answer the question it was written for. So he always asks which facts are necessary before he asks anyone to find them. The GMAT, he added, tests exactly this skill, and calls it Data Sufficiency.