At a speed of 50 miles per hour, a certain car

[sara.leighton]

At a speed of 50 miles per hour, a certain car uses 1 gallon of gasoline every 30 miles. If the car starts with a full 12 gallon take of gasoline and travels for 5 hours at 50 miles per hour, the amount of gasoline used would be what fraction of a full tank?

A) 3/25
B) 11/36
C) 7/12
D) 2/3
E) 25/36 => ANSWER

How do you get the correct answer? I calculate the car has traveled 50*5=250 miles * 1 gallon gas/30 miles = 8.3 gallons used, 8.3/12 = ~2/3.

[saurav.raaj]

Sara,

You have the correct answer, but incorrect approximation.

You should not decimal-ise 250/30 = 8.3, instead leave it as 25/3.

When you divide it by 12 again, you get 25 / 36.

[RonPurewal]

ah, yes.

NEVER approximate ANYTHING unless the answer choices are fairly far apart. when the answers are as close together as 2/3 and 25/36 (which are very close together indeed), the difference between the approximation and the true value could mean the difference between the right answer and the wrong answer.

[pratik.munjal]

Well, this problems starts off as an easy one, but gets a little daunting towards the end. Luckily I got it right in the prep test.

Here's what I did.

Speed=50 miles/hour (given)
Time=5 hours (given)

So lets start by calculating the distance travelled.

D = S x T

D= 50 x 50=250 miles

Now, using the unitary method:

30 miles --> 1 gallons
1 mile(s)--> 1/30 gallons
250 miles --> 1/30 x 250 gallons

This comes to 25/3.

Total fuel=12 (given in question). To find the answer divide fuel used by total fuel.

(25/3)/12=25/36. And that's the answer. Hope this seemed easier.

(This is exactly how I did it in the prep test-although the process seems more untrammeled now. But I guess getting questions right under test conditions is daunting)

[jnelson0612]

Nice, pratik! I like your method.

[sadat.sathak]

Another approach:

The question is asking us how much of the full tank was consumed.

The tank's full capacity is 12 litres and since each litre gives the mileage of 30 miles, on a full tank @ 50mph you could travel 360 miles (12*30)

What does the question tell us? Ah yes, the car travelled 5 hours at 50 miles=250 miles (50*5)

Therefore the fractions used is 250/360...Zeroes drop of and voila=> 25/36

[tim]

thanks..