Explanation:

- The shopkeeper applies a 30% discount to the marked price of Rs 480.

- This means the selling price is 70% of Rs 480: $0.7\times 480=336$.

- The selling price allows a 20% gain over the cost price.

- If the cost price is $x$, then the selling price of Rs 336 corresponds to 120% of the cost price: $1.2x=336$.

- Solving for $x$, gives $x=\frac{336}{1.2}=280$.

- Therefore, the cost price of the article is Rs 280.

Option 4 - Rs 280