At least three times. The first, fifth and third groups. Weigh two groups 10 at the same time. There will be two situations, balanced or unbalanced. It won't happen again. Balance, the remaining five are defective. Unbalanced, the lower tray is defective. The next step is to select the defective products from the five. In pairs, put four on the plate and leave one. There are still only two situations. Balance and imbalance. Balance, leaving one defective. Imbalance. Defective products in the lower tray. Take it out and weigh it for the third time. The lower tray is defective. End.