A merchant marks his goods at Rs. 600 and allows a discount of 25%. If he still gains 11.11%, then the cost price of the article is:

Step 1: Calculate the selling price after discount

The merchant allows a discount of 25% on the marked price of Rs. 600.

So, the discount amount = 25/100 * 600 = Rs. 150

Therefore, the selling price after discount = Marked price - Discount = 600 - 150 = Rs. 450

Step 2: Calculate the cost price

The merchant still gains 11.11% on the cost price.

So, if 111.11% (100% cost price + 11.11% gain) of the cost price is equal to the selling price, we can set up the following equation to find the cost price:

111.11/100 * Cost price = Selling price

Substituting the selling price from step 1:

111.11/100 * Cost price = 450

Solving for Cost price:

Cost price = 450 / (111.11/100) = Rs. 405

So, the cost price of the article is Rs. 405.