A fruit seller sold $20$ apples for $\text{Rs. }50$ and made a profit of $25\%$. How much did he spend on buying $20$ apples?

Let the cost price of $20$ apples be $\text{CP}$.
Given parameters:

* Selling price ($\text{SP}$) of $20$ apples = $\text{Rs. }50$
* Profit Percentage ($\text{P}\%$) = $25\%$

Using the profit relation formula:

$$\text{SP}=\text{CP}\times \left(1+\frac{\text{P}\%}{100}\right)$$

$$50=\text{CP}\times \left(1+\frac{25}{100}\right)$$

$$50=\text{CP}\times 1.25$$

Solve for $\text{CP}$:

$$\text{CP}=\frac{50}{1.25}=\frac{5000}{125}=\text{Rs. }40$$

Therefore, he spent $\text{Rs. }40$ on buying the apples.