If the cost price of $30$ tables is equal to the selling price of $40$ tables, then what is the loss percentage?

Let the Cost Price ($\text{CP}$) of $1$ table be $\text{CP}$ and the Selling Price ($\text{SP}$) of $1$ table be $\text{SP}$.
According to the question:

$$30\times \text{CP}=40\times \text{SP}$$

Rearranging the terms to find the ratio of $\text{CP}$ to $\text{SP}$:

$$\frac{\text{CP}}{\text{SP}}=\frac{40}{30}=\frac{4}{3}$$

This implies that if the Cost Price is $4$ units, the Selling Price is $3$ units.

Since $\text{CP}>\text{SP}$, there is a loss:

$$\text{Loss}=\text{CP}-\text{SP}=4-3=1\text{ unit}$$

Now, calculate the loss percentage:

$$\text{Loss Percentage}=\left(\frac{\text{Loss}}{\text{CP}}\times 100\right)\%$$

$$\text{Loss Percentage}=\left(\frac{1}{4}\times 100\right)\%=25\%$$

Therefore, the loss percentage is $25\%$.