You want to calculate one half of one half, or:
[tex]\sf\dfrac{1}{2}\times\dfrac{1}{2}[/tex]
Multiply the numerators and denominators together:
[tex]\sf\dfrac{1\times 1}{2\times 2}[/tex]
[tex]\sf\dfrac{1}{4}[/tex]
So each friend gets 1/4 of a whole pineapple.