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%?

Seller uses faulty weight instead of 2 kg i.e., 2000 gram
Gain = 25%
Let's say error = $$a$$
$$\eqalign{ & {\text{Profit }}\% = \frac{{{\text{Error}}}}{{{\text{True weight}} - {\text{Error}}}} \times 100 \cr & 25 = \frac{a}{{2000 - a}} \times 100 \cr & \frac{1}{4} = \frac{a}{{2000 - a}} \cr & 2000 - a = 4a \cr & 2000 = 5a \cr & {\text{So, }}a = 400{\text{ gm}}{\text{.}} \cr} $$