Question

Your car has a 15 gallon tank and can run on fuel with added ethanol. The fuel contains either 15 percent ethanol or 65 percent ethanol. How much of 65% percent ethanol fuel should you use so that your tank is 40 percent ethanol?

Answer 1

Total: 15 gallons

Let x be the fuel with 15% ethanol and y be the one wit 65% ethanol

x+y = 15

0.15x+0.65y=0.4*15 = 6

So, let's isolate x in the first equation:

x=15-y

And substitute on the second:

0.15(15-y) +0.85y=6

2.25y+0.65y = 6

2,9y=6

y=2.06 gallons

x= 12.94 gallons

I should use 2.06 gallons of 65% ethanol.