fraction question

[tj123]

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

I chose 2/3.

The answer is 25/36

Please explain how to get the answer.

[mental]

At a speed of 50mph, a certain car uses 1 gallon of gasoline every 30 miles.

travels for 5 hours at 50 mph

distance travelled: 5*50 = 250 miles

gas used: 250/30 = 25/3 gallon = x

the amount of gasoline used would be what fraction of a full tank?

x is what fraction of 12?

x/12 = (25/3)/12 = 25/36

[tj123]

i did 250/30 = 8.33

then i did 8/12 which = 2/3

why is this way wrong?
i used the same method as you used?

[Stuart@KaplanGMAT]

i did 250/30 = 8.33

then i did 8/12 which = 2/3

why is this way wrong?
i used the same method as you used?

Because you did 8/12 instead of 8.33/12.

Let's convert 8.33 to a fraction to make the math easier.

8.33 = 8 and 1/3 = 25/3

(25/3)/12 = (25/3)*(1/12) = 25/36.

25/36 is only 1/36 more than 2/3. When the answers are that close together, it's not safe to round off.

[niraj_a]

stuart,

this was certaonly helpful, but would a nitty-gritty on these lines show up on the real GMAT?

[canuckclint]

stuart,

this was certaonly helpful, but would a nitty-gritty on these lines show up on the real GMAT?

GMAT is multiple choice.
If they do not say to approximate then do not.

They will provide a fractional answer like 25/36 and you can chose that.
They will not check your work.

[niraj_a]

canuck,

2/3 is a fraction too right? no issues, i'll look out for the word 'approximate' and then be sure to choose the most BS answer that's close to my calculated answer.