2 pizzas were ordered and had 12 slices each for a total of 2*12 = <<2*12=24>>24 slices
Dean ate 1/2 of a 12 sliced pizza so 12/2 = <<12/2=6>>6 slices were left over
Sammy ate 1/3 of a 12 sliced pizza so 12/3 = <<12/3=4>>4 slices were left over
Dean ate 6 slices, Sammy ate 4 and Frank had 3 so 6+4+3 = <<6+4+3=13>>13
They started with 24 slices and 13 slices so 24-13 = <<24-13=11>>11 slices left
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