The constant of proportionality is the constant value of the ratio of y and x. It is usually denoted as k.
k = y/x
OR
y = kx
From the information given, the value of k is derived as;
[tex]\begin{gathered} k=2\frac{1}{2}\text{ / }\frac{1}{2} \\ k=\frac{5}{2}\times\frac{2}{1} \\ k=5 \end{gathered}[/tex]
Where y = kx, then
[tex]y=5x[/tex]
That means, the value of y can always be derived by multiplying x by the constant of proportionality
