For the information given, you know that the cost for 1 mile is
[tex]\frac{\text{ \$}40.50}{15}=\text{ \$2.7}[/tex]That is, the constant of proportionality is 2.7.
For point 2) the equation that represents this situation is
[tex]\begin{gathered} C(x)=2.7x \\ \text{ Where x is the number of miles and} \\ C(x)=\text{ cost of a trip of x miles} \end{gathered}[/tex]For point 3) If you take x = 20 miles you have
[tex]\begin{gathered} C(x)=2.7x \\ C(20)=2.7(20) \\ C(20)=\text{ \$54} \end{gathered}[/tex]That is, the cost of a 20 mile trip in this taxi is $54.