A fruit seller professes to sell his fruits at cost price, but still gains 25% on his outlay. What weight does he substitute for a kilogram ?

$$\eqalign{ & {\text{Let error}} = x{\text{ gms}}{\text{.}} \cr & {\text{Then,}} \cr & = \frac{x}{{1000 - x}} \times 100 = 25 \cr & \Leftrightarrow \frac{{100x}}{{1000 - x}} = 25 \cr & \Leftrightarrow 100x = 25000 - 25x \cr & \Leftrightarrow 125x = 25000 \cr & \Leftrightarrow x = 200 \cr & \therefore {\text{Weight used}} \cr & = (1000 - 200){\text{gms}} \cr & = {\text{800 gms}}. \cr} $$