The first step is to calculate the total number of pizza slices there are.
We know each slice is 1/8 of the pizza. (think of 1/8 as 1 out of 8 pieces) This means each pizza has 8 slices. There are 3 pizzas so there are 8 x 3 = 24 slices total
Next we want to figure how many slices of pizza they ate which will help us figure out how many are left.
They ate 3/4 of the pizza, so in other words only 1/4 of the pizza is left.
In order to figure out how many slices are left, all we need to do is find out what 1/4 of the total number of slices are.
# of slices left = Total number of slices x 1/4 (1/4 of the pizza is left)