By selling a car for $\text{₹ }6,400$, Mr. Rao lost $20\%$. Then the cost price of the car is:

Given parameters:
* Selling Price ($\text{SP}$) = $\text{₹ }6,400$
* Loss Percentage ($\text{Loss}\%$) = $20\%$

The relation between Cost Price ($\text{CP}$) and Selling Price ($\text{SP}$) during a loss is:

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

$$6400=\text{CP}\times \left(1-\frac{20}{100}\right)$$

$$6400=\text{CP}\times 0.80$$

$$\text{CP}=\frac{6400}{0.80}=\frac{64000}{8}=\text{₹ }8,000$$