A seller uses faulty weight in place of a 2 kg weight and earns a 25\% profit. He claims that he is selling on the cost price in front of the customers but uses a faulty weight. How much error is there in the 2 kg weight to gain 25\%?

Let actual weight = x kg. Shopkeeper uses 2 kg weight but gives only x kg. He claims to sell at cost price but makes 25% profit. This means: Selling price (for 2 kg) = Cost price (for x kg) × 1.25. Since selling price = cost price (he claims no profit), we have: 2 kg price = x kg price × 1.25. So x = 2/1.25 = 1.6 kg. Error = 2 kg - 1.6 kg = 0.4 kg = 400 gm.