i brought 12 products. and i am getting 20% discount on it. and i am getting 1 product for free. So what is percentage of my total discount?
If all products are the same price p, you are paying 0.8*12p, but at full price for all 13 you would pay 13p.
So the discount is (1 - 0.8*12p/13p)*100% ≈ 26.15%