Jatin buys an article for $10\%$ less than its value and sells it for $10\%$ more than its value. His gain is

Let the true actual value of the article be $\text{Rs. }100$.Cost Price ($\text{CP}$): Jatin buys it for $10\%$ less than its true value.
$$\text{CP}=100-(10\%\text{ of }100)=100-10=\text{Rs. }90$$
Selling Price ($\text{SP}$): Jatin sells it for $10\%$ more than its true value.
$$\text{SP}=100+(10\%\text{ of }100)=100+10=\text{Rs. }110$$
Profit Calculation:
$$\text{Total Profit}=\text{SP}-\text{CP}=110-90=\text{Rs. }20$$
Gain Percentage ($\text{Gain }\%$):
$$\text{Gain }\%=\left(\frac{\text{Profit}}{\text{CP}}\right)\times 100$$
$$\text{Gain }\%=\left(\frac{20}{90}\right)\times 100=\frac{200}{9}=22\frac{2}{9}\%$$