A shopkeeper by selling $9$ Rolex watches, earns a profit equal to the selling price of $4$ Rolex watches. His profit percentage is:

Let the Selling Price ($\text{SP}$) of $1$ watch be $\text{Rs }1$.Then, the Selling Price of $9$ watches = $\text{Rs }9$.According to the question, the profit earned is equal to the selling price of $4$ watches:
$$\text{Profit}=\text{Rs }4$$
We know that:
$$\text{Cost Price (CP)}=\text{Selling Price (SP)}-\text{Profit}$$
$$\text{CP}=9-4=\text{Rs }5$$
Now, calculate the profit percentage:
$$\text{Profit Percentage}=\left(\frac{\text{Profit}}{\text{CP}}\right)\times 100\%$$
$$\text{Profit Percentage}=\left(\frac{4}{5}\right)\times 100\%=80\%$$