The proportional equation for the the number of gallons of yellow paint, y needed for any amount of blue paint, b is:
[tex]y(b) = ab[/tex]
In this problem, the parameters are:
Then, to find the constant of proportionality:
[tex]y(b) = ab[/tex]
[tex]\frac{4}{5} = \frac{1}{2}a[/tex]
[tex]a = \frac{8}{5}[/tex]
Hence, the equation is:
[tex]y(b) = \frac{8}{5}b[/tex]
You can learn more about proportional equation at https://brainly.com/question/13112448