Explanation:

To find the original cost price, we need to reverse the percentage changes applied to the final price.

- Let's assume the original cost price is Rs. X.

- First, it's decreased by 10%: $X-0.1X=0.9X$.

- Then it's increased by 20%: $0.9X+0.18X=1.08X$.

- The final price, after these changes, is Rs. 540.

So, we set up the equation:

$$1.08X=540$$

Solving for X:

- Divide both sides by 1.08:

$$X=\frac{540}{1.08}=500$$

- Option 1: Rs. 500 is indeed the original cost price.

Option 1: Rs. 500