The special offer in Buy 2 get 1 free, the the discount will be 33% off.
Or say $40 for 2, and the original price is $21.8@, then the discount is 9.17% off.
There is offer in buy a 2L cola by $11, while a pack of 8 with @355ml is $16, then by volume, the first costs $5.5/L, while $5.634/L for the second.
and etc.