start by finding the y intercept, x=0
[tex]\begin{gathered} y=\frac{15+3x}{3+x} \\ y=\frac{15}{3}=5 \end{gathered}[/tex]meaning that the y-intercept is equal to $5
since the function represents the average cost and x=0 means that the y intercept represents the average cost per pint of all the raspberries he bought.