First find how much barbecue sauce Jack makes by adding the volume of each ingredient: 3 cups + 1 cup + 1 cup = <<3+1+1=5>>5 cups
Then multiply the number of pulled pork sandwiches by the amount of sauce per sandwich to find the total amount used: 1/6 cup/sandwich * 18 sandwiches = <<1/6*18=3>>3 cups
Then subtract this amount from the total amount of sauce Jack has to find how much he uses on the burgers: 5 cups - 3 cups = <<5-3=2>>2 cups
Then divide this amount by the amount of sauce used per burger to find the number of burgers he can make: 2 cups / 1/4 cup/burger = 8 burgers
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