There are 5 people (Sarah and her 4 friends):
[tex]p=5[/tex]Each one of them gets 3 cookies:
[tex]s=3[/tex]If c is the number of cookies, we have the equation of the situation:
(# of cookies)/(# of people) = (# of shared cookies)
Finally:
[tex]\begin{gathered} \frac{c}{p}=s \\ \\ \therefore\frac{c}{5}=3 \end{gathered}[/tex]