How many gallons of e85 fuel should kevin use if he wants to fill up his car with 10 gallons of fuel containing 25% ethanol?

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.