Let the marked price of the article be x.
First a 20% discount was offered, on which another 25% discount was offered.
So, after applying discounts, the customer paid 75% of 80% of marked price, which is equal to Selling price
=> 75% of 80% of x = 3600
=> (75/100) * (80/100) * x = 3600
=> x = 3600 * (5/3)
=> x = 6000
So, the marked price of the article = Rs. 6000
Formula:- Cost price of the article = {100 / (100 + Gain %)} * S.P
= [100/(100+80)]*3600
= [100 / 180] * 3600
= Rs. 2000