A shopkeeper sold a product at loss of 10%. He had sold it for Tk. 108 more, he would have earned a profit of 10%. Find the cost of the product.

At loss of 10%, if cost price is Tk. 100, selling price is Tk. (100 - 10) = Tk. 90
At profit of 10%, if cost price is Tk. 100, selling price is Tk. (100 + 10) = Tk. 110
More selling price = Tk. (110 - 90) = Tk. 20
if more selling price is Tk. 20, the cost price is Tk. 100
if more selling price is Tk. 108, the cost price is Tk. 100 × 108/20 = Tk. 540
-------------------------------------------------------------------------------------
Shortcut way:
more selling price = 10 - ( - 10) = 20
Therefore, 20% of cost price = More selling price
20% of cost price = 108
cost price = Tk. 100 × 108/20 = Tk. 540