25% on cost will get you the same profit. Explanation is as follows.
When profit % is expressed on an item, then such item is assumed to be 100% (i-e If profit is expressed as 15% on cost, then, cost is assumed as 100 (any currency unit) . If profit is expressed as 5% on Selling price, then, Selling price is 100 (any currency unit).
In the present case, 20% profit is on Selling price. Hence, by applying the above logic, Selling price is INR 100. Profit would then be INR 20.00 (20% on INR 100).
So, to earn the same INR 20 profit, the profit percentage on cost is calculated as below ;
Cost = 80
Profit = 20
Hence, profit % on cost is 20/80 = 0.25 or 25%.
Hope this helps.