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

Let the Cost Price (CP) of the car be $x$.
Given:

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

The relation between SP and CP when there is a loss is:

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

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

$$6400=x\times \frac{80}{100}$$

$$x=\frac{6400\times 100}{80}$$

$$x=80\times 100=\text{₹}8,000$$

Therefore, the cost price of the car is $\text{₹}8,000$.