Data Sufficiency

Jay@ManhattanReview wrote:

lheiannie07 wrote:The amount of coal a train burns each mile is directly proportional to the speed at which it travels. How much coal will it burn on this particular 60 mile trip?

(1) On a previous trip, the train burned 100 pounds of coal on a 60 mile trip at 60 miles per hour
(2) On this particular trip, the train is travelling at a speed of 30 miles per hour.

Isn't the second statement irrelevant?Why isn't Option A the best Option?

OA C

Say A = Amount of coal burned, S is the speed and k is a constant of proportionality

Thus, A = Sk. we have to get the value of A. If we get S and k, we get the answer.

(1) On a previous trip, the train burned 100 pounds of coal on a 60 mile trip at 60 miles per hour.

=> 100 = 60k => k = 100/60 = 5/3. But we do not know at what speed the train will run on that perticular trip. Insufficient.

(2) On this particular trip, the train is traveling at a speed of 30 miles per hour.

Though we know the value of S, we do not know the value of k. Insufficient.

(1) and (2) combined:

From (1) we have k = 5/3 and from (2) we have S = 30.

Thus, A = 30*(5/3) = 50 pounds. Sufficient.

The correct answer: C

Hope this helps!

-Jay
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