A hawker purchases some mangoes at the rate of $9$ mangoes for $\text{Rs. }5$ and sells them at $\text{Rs. }8$ per dozen. What will be his profit or loss percentage?

Cost Price ($\text{CP}$) of 9 mangoes = $\text{Rs. }5$
$$\text{CP of 1 mango}=\frac{5}{9}$$

Selling Price ($\text{SP}$) of 1 dozen (12 mangoes) = $\text{Rs. }8$

$$\text{SP of 1 mango}=\frac{8}{12}=\frac{2}{3}$$

To compare, find the profit or loss per mango:

$$\text{Profit}=\text{SP}-\text{CP}=\frac{2}{3}-\frac{5}{9}=\frac{6-5}{9}=\frac{1}{9}$$

Since the value is positive, it is a profit. Now, calculate the profit percentage:

$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{CP}}\right)\times 100=\left(\frac{\frac{1}{9}}{\frac{5}{9}}\right)\times 100=\frac{1}{5}\times 100=20\%$$

Thus, the hawker achieves a $20\%$ profit.