Question 1130048
<pre>
Average fuel price of  $3.00 per gallon


          Fuel Effic MPG       Dollars per Mile    Distance MILES    COST dollars

City        28                 3/28                  x               (3/28)x

Highway     39                 3/39                  y               (3/39)y

Total                                               1543
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The cost of the trip depends on two variables for distances x and y.
Total cost:   {{{(3/28)x+(3/39)y}}}


You could take this another step and use {{{x+y=1543}}}
{{{y=1543-x}}}


substituting that,
{{{(3/28)x+(3/39)(1543-x)}}}----and simplify this if you want, and have the cost  in terms of the city miles distance, keeping the constant total trip 1543 miles.