Dilnoza runs the office of a small consulting firm in Tashkent. Every month a software supplier sends an invoice, and every month someone pays it without reading it.
This month Dilnoza read it. The firm had bought licenses for 198 employees at 25 dollars per license. The invoice asked for 5,500 dollars.
She did not reach for a calculator. Two hundred licenses at 25 dollars would be 5,000 dollars, she thought, because 25 is a quarter of 100. The firm had bought two fewer, so the real figure was 50 dollars less: 4,950 dollars in total. Even without the correction, the invoice was clearly too high.
She called the supplier. A clerk had typed 220 licenses instead of 198. The corrected invoice arrived the same afternoon.
Dilnoza then checked the second line of the invoice: 36 hours of technical support at 45 dollars per hour. She split the 36 hours into 30 and 6. Thirty hours cost 1,350 dollars and six hours cost 270, so the support came to 1,620 dollars. That line was right.
Her manager asked how she had found the error so quickly. "I didn't calculate it exactly at first," she said. "I only asked which number the answer should be closest to. When the invoice was far from that, I knew something was wrong."
Since then, the firm checks every invoice the same way: estimate first, then calculate the lines that look suspicious.