Some article were bought at 11 for Rs. 10 and sold at 10 for Rs. 11. Gain percent is           

CP of  11 articles = 10/-

CP of  1 article = $\frac{10}{11}$

 SP of  10 articles = 11

SP of  1 article = $\frac{11}{10}$

$\text{Profit\hspace{0.17em}\hspace{0.17em}\%\hspace{0.17em}\hspace{0.17em}=\hspace{0.17em}\hspace{0.17em}}\frac{\text{SP-CP}}{\text{CP}}\text{×100}$

$\text{=}\frac{\frac{\text{11}}{\text{10}}\text{-}\frac{\text{10}}{\text{11}}}{\frac{\text{10}}{\text{11}}}\text{×100}$

$\text{=}\frac{\text{21×11}}{\text{10×10}}\text{×100=21\%}$