Answer:
He should use 2 gallons of E85.
Step-by-step explanation:
Let x = amount of E85 fuel
Let y = amount of 10% ethanol fuel.
The tank holds 10 gallons, so the gallon equation is
x + y = 10
Now we write an equation for the percent ethanol.
0.85x + 0.1y = 0.25(x + y)
0.85x + 0.1y = 0.25x + 0.25y
0.6x - 0.15y = 0
Solve the volume equation for y:
y = 10 - x
Substitute in the ethanol equation.
0.6x - 0.15y = 0
0.6x - 0.15(10 - x) = 0
0.6x - 1.5 + 0.15x = 0
0.75x = 1.5
x = 1.5/0.75
x = 2
He should use 2 gallons of E85.
