Answer:
The price per gallon of gas with a $7 car wash is $3.19
Then if you purchase x gallons of gas and also pay for the car wash, the total cost is:
C(x) = $7 + $3.19*x
In the case that you do not pay for the car wash, the cost per gallon is $3.39
Then the cost for x gallons of gas in this case is:
K(x) = $3.39*x
When is it worth it to buy the car wash?
We need to sett the equations:
C(x) = K(x)
This means that you pay exactly the same in both cases, but in one ( C(x)) you get the car wash and in the other not.
Then we need to solve:
$7 + $3.19*x = $3.39*x
$7 = ($3.39 - $3.19)*x = $0.20*x
$7/$0.20 = x = 35
Then for 35 gallons, is worth to buy the car wash.
In the case where the car wash costs $2, then the equation C(x) becomes:
C(x) = $2 + $3.19*x
Then if we solve the same equation:
C(x) =K(x)
$2 + $3.19*x = $3.39*x
$2 = ($3.39 - $3.19)*x
$2 = $0.20*x
$2/$0.20 = x
10 = x
Then for 10 galons is whort buy the car wash.