A certain military vehicle can run on pure Fuel X, pure Fuel Y, or any mixture of X and Y. Fuel X costs $3 per gallon; the vehicle can go 20 miles on a gallon of Fuel X. In contrast, Fuel Y costs $5 per gallon, but the vehicle can go 40 miles on a gallon of Fuel Y. What is the cost per gallon of the fuel mixture currently in the vehicle’s tank?
1) Using fuel currently in its tank, the vehicle burned 8 gallons to cover 200 miles.
2) The vehicle can cover 7 and 1/7 miles for every dollar of fuel currently in its tank

most important facts concerning such averages is the following: in taking a weighted average of 2 quantities, knowing the RATIO of the quantities is equivalent to knowing the VALUE OF THE AVERAGE itself.

Stmt 1 )
this is equivalent to giving the miles per gallon for the fuel mixture: 200/8 = 25 miles per gallon. fuel x gets 20 miles/gallon, and fuel y gets 40 miles/gallon.
By using the concept of Wiegthed average, we can obtain the ratio of x:y
X/y = 40 - 25/ 25-20
Using the ratio, one can calulate the cost per gallon of the fuel mixture as :X/Y = 5-x/x-3

Stmt 2)
We can find the miles covered for every dollar of X and Y and then using WA, we can find the ratio of X/Y. Hence Suff.
Ans : D