Question 708854
"What percent of the mixture", means for 100 parts.  First set up the cost equation.


x and y, the gallons or just parts of each fuel to mix.  x the fuel, y the substitute.


{{{2.48x+1.82y=2.15}}} dollars  (so this uses x and y as gallons).  That is for 1 gallon of mixture.


Since that was for just 1 gallon, we expect x+y=1.  You can imagine the 1 as made of 100 equal parts; the gallon can be understood as divisible into 0.01 multiplied by 100.


We just have this system:
{{{2.48x+1.82y=2.15}}}
{{{x+y=1}}}


Multiply the second equation by 1.82, and subtract it from the first equation and get a value for x (after a little more arithmetic).

2.48x-1.82x+1.82y-1.82y=2.15-1.82(1)
0.66x=0.33
{{{highlight(x=0.5)}}}

That means y is also {{{highlight(0.5)}}}


ANSWER: Half gallon for each one to make 1 gallon of fuel mixture at 2.15 dollars per gallon.  50% of each fuel.