S.P. of $300$ articles is equal to C.P. of $500$ articles. Find the loss or profit percentage.

Let the Cost Price (C.P.) of $1$ article be $\text{Rs. }1$.
Then, C.P. of $500$ articles $=\text{Rs. }500$.
According to the problem, Selling Price (S.P.) of $300$ articles $=$ C.P. of $500$ articles $=\text{Rs. }500$.

Now, let's find the C.P. of the same $300$ articles:
C.P. of $300$ articles $=\text{Rs. }300$.

Since $\text{S.P.}>\text{C.P.}$, there is a profit:

$$\text{Profit}=\text{S.P.}-\text{C.P.}=500-300=200$$

Now, calculate the profit percentage:

$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{C.P. of 300 articles}}\right)\times 100\%$$

$$\text{Profit Percentage}=\left(\frac{200}{300}\right)\times 100\%=\frac{200}{3}=66\frac{2}{3}\%$$