If you buy X number of refrigerator filters you also gain X number of free refrigerator filters. That means you will gain 2X numbers of refrigerator filters for the price of 1X numbers of it.
For example:
If you buy 2 refrigerator filters you also gain 2 free refrigerator filters. In total 4 refrigerator filters.
If you buy 3 refrigerator filters you also gain 3 free refrigerator filters. In total 6 refrigerator filters.
If you buy 4 refrigerator filters you also gain 4 free refrigerator filters. In total 8 refrigerator filters.
