You start by answering one question at a time: what's the buying cycle for this product type, whether each customer's last-purchase date is on record, and what no-purchase window the store uses today for this product (30, 60, 90 days, or none). The tutor uses your answers to figure out if the problem is missing records or the wrong number.
It compares the logic to an oil change: change it too early and you waste good oil, change it too late and you damage the engine, and the right point depends on the car, not a fixed number of miles. The same way, a short-cycle product calls for 30 days, a medium-cycle one for 60, and a longer-cycle one for 90. If the store uses 60 days for everything, including fast-turnover products, the tutor shows why that leaves the store noticing who stopped far too late.
At the end, it asks the store to walk back the reasoning in its own words: this product has a cycle of this length, so this is the window that matters, and a smaller number would catch people who were still going to come back while a bigger one would take too long to notice. When the store returns, the tutor asks which window it picked for that product and who showed up on the list, fixing only the number or the date reading, without repeating the whole lesson.