Each pizza is 12 slices. Bob at 12 slices / 2 = <<12/2=6>>6 slices.
Tom ate one third, or 12 slices / 3 = <<12/3=4>>4 slices.
Sally ate one sixth, or 12 slices / 6 = <<12/6=2>>2 slices.
Jerry ate one quarter, or 12 slices / 4 = <<12/4=3>>3 slices.
In total, they ate 6 slices + 4 slices + 2 slices + 3 slices = <<6+4+2+3=15>>15 slices.
So they were left with 12 slices per pizza * 2 pizzas - 15 slices = 24 slices - 15 slices = <<12*2-15=9>>9 slices.
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