Left pan of a faculty balance weighs 100 gms more than its right pan. A shopkeeper keeps the weight measure in the left pan while buying goods but keeps it in the right pan while selling his goods. He uses only 1 kg weight measure. If he sells his goods at the listed cost price, what is his gain ?

Explanation:-

Let the C.P. of 1 kg goods be Rs. 1
Then,
He buys 1100 g goods for Rs. 1 and sells 900 g goods for Rs. 1
∴ C.P. of 1100 g goods = Rs. 1
⇒ C.P. of 900 g goods $$\eqalign{ & = {\text{Rs}}.\left( {\frac{1}{{1100}} \times 900} \right) \cr & = {\text{Rs}}.\frac{9}{{11}} \cr}$$ S.P. of 900 g goods = Rs. 1 $$\eqalign{ & {\text{Gain = Rs}}.\left( {1 - \frac{9}{{11}}} \right) \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\, = {\text{Rs}}{\text{.}}\frac{2}{{11}}. \cr & \therefore {\text{Gain }}\% \cr & = \left( {\frac{2}{{11}} \times \frac{{11}}{9} \times 100} \right)\% \cr & = \frac{{200}}{9}\% . \cr}$$