A shopkeeper cheats to the extent of 10% while buying as well as selling, by using false weights. His total gain is -

$$\eqalign{ & {\text{Rule}}:{\text{Gain}}\% \cr & = \frac{{{{\left( {100 + {\text{common gain}}\% } \right)}^2}}}{{100}} - 100 \cr & \therefore {\text{Gain }}\% \cr & = \left[ {\frac{{{{\left( {100 + 10} \right)}^2}}}{{100}} - 100} \right]\% \cr & = \left( {\frac{{12100 - 10000}}{{100}}} \right)\% \cr & = 21\% \cr} $$