81 dollars
Explanation
you can easily solve this by using a rule of three
Step 1
Let x represents the cost for 900 miles ( the whole road trip)
if
[tex]\begin{gathered} 54\text{ dollars}\Rightarrow600\text{ miles} \\ \text{then} \\ x\text{ dollars}\Rightarrow900\text{ miles} \end{gathered}[/tex]as the proportion is the same
[tex]\begin{gathered} \frac{54}{600}=\frac{x}{900} \\ \text{Multiply both sides by 900} \\ \frac{54}{600}\cdot900=\frac{x}{900}\cdot900 \\ 81=x \end{gathered}[/tex]I hope this helps you
