[tex]C = 7.2b[/tex]
From the question we have that,
27 boxes --------------- $194.40
b boxes --------------- C
Using the Rule of Three we have that,
[tex]27C = 194.40b[/tex]
Since we want a function that represents C in terms of b ( C = f(b) ) we can isolate C on the equation,
[tex]C = \frac{194.40}{27}b = 7.2b[/tex]