Solution
Harry buys 3 granola bars for $5. Sidnee buys 9 granola bars for $15.
[tex]\begin{gathered} y=kx \\ \end{gathered}[/tex]Where
x=3
y=5
k is the proportionality
[tex]\begin{gathered} 5=3k \\ divide\text{ both sides both sides} \\ \frac{5}{3}=\frac{3k}{3} \\ \\ k=\frac{5}{3} \\ \\ k=1.6667 \\ \end{gathered}[/tex]The constant proportionality is
[tex]1.6667[/tex]