A fruit seller purchased bananas at $\text{Rs. }30$ per dozen and sold each pair at $\text{Rs. }7$. What is the percentage profit or loss at this?

1 dozen = 12 items.
Cost Price ($\text{CP}$) of 12 bananas = $\text{Rs. }30$.

$$\text{CP of 1 banana}=\frac{30}{12}=\text{Rs. }2.5$$

1 pair = 2 items.
Selling Price ($\text{SP}$) of 2 bananas = $\text{Rs. }7$.

$$\text{SP of 1 banana}=\frac{7}{2}=\text{Rs. }3.5$$

Since $\text{SP}>\text{CP}$, the seller makes a profit:

$$\text{Profit per banana}=3.5-2.5=\text{Rs. }1.0$$

Calculate profit percentage:

$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{CP}}\right)\times 100=\left(\frac{1.0}{2.5}\right)\times 100=40\%$$

Hence, the fruit seller makes a $40\%$ profit.