Every day, Billie bakes 3 pumpkin pies for 11 days and puts them in the refrigerator.  It takes 2 cans of whipped cream to cover 1 pie.  If Tiffany comes over and eats 4 pies, how many cans of whipped cream does Billie need to buy to cover the remaining pies?
Please reason step by step, and put your final answer within \boxed{}.
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