Hi jjjinapinch,

We're told that a truck uses 1/12 + (K)(V^2) gallons of fuel per mile when its speed is V miles per hour and that K is a constant.
We're asked for the speed at which the truck travels so that it uses 5/12 gallon of fuel per mile.

To start, we can input the specific 'fuel usage' that the question asks us to deal with into the given equation:

5/12 = 1/12 + (K)(V^2)

Thus, to figure out the speed (V), we need to know the constant (K).

1) The value of K is 1/10,800

Fact 1 gives us the constant, we can figure out the value of V (and since we can't have a "negative speed", there's only one value for V^2 - the positive one).
Fact 1 is SUFFICIENT

2) When the truck travels at 30 miles per hour, it uses 1/6 gallon of fuel per mile.

We can plug this information into the equation, which gives us...

1/6 = 1/2 + (K)(30^2)

With this equation, we can then solve for K (which we could then plug back into the original question and solve for V).
Fact 2 is SUFFICIENT

Final Answer: D

GMAT assassins aren't born, they're made,
Rich