He ordered 2 pizzas, each cut into 12 slices so that’s 2*12 = <<2*12=24>>24 slices
Stephen ate 25% of the pizza so he ate .25*24 = <<25*.01*24=6>>6 slices
That leaves 24-6 = <<24-6=18>>18 slices
18 slices are left and Pete eats 50% of 18 slices so he eats 18*.50 = <<18*50*.01=9>>9 slices
If there were 18 slices and Pete ate 9 slices then there are 18-9 = <<18-9=9>>9 slices left over
#### 9