4 friends evenly dividing up a n - slice pizza
So each one get [tex]\frac{n}{4}[/tex] slices of pizza.
For example, if n = 12 slice then each friend gets 12/4 = 3 pieces. If Harris ate one slice less then he ate [tex]\frac{12}{4}-1[/tex]=3 -1 = 2 pieces.
So the general expression for slices of pizza did Harris eat is
[tex]\frac{n}{4}-1[/tex]