Question 948418
That is the same form of fuel efficiency problem as asked in this example which has been solved:


<a href="http://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.948009.html">http://www.algebra.com/algebra/homework/coordinate/word/Linear_Equations_And_Systems_Word_Problems.faq.question.948009.html</a>


Study that problem and solution.  If you understand it, then you should be able to solve yours.