The ratio of cost price and selling price of an article is 20 : 21. The gain percent on it is = ?

Answer: Option B

According to the question,
$$\begin{array}{rl}& \frac{\text{CP}}{\text{SP}}=\frac{20}{21})+1=\text{Profit}\\ & \text{Profit }\%=\frac{1}{20}\times 100\\ & =5\%\end{array}$$