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Reading Questions Incorrectly.... Why? Need advice...

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Reading Questions Incorrectly.... Why? Need advice...

by mkbigmoz » Tue Mar 10, 2009 1:00 pm
I will post a question which I missed from the Kaplan CATs. I will also post my explanation on how I interpreted the question. Im looking for advice on how I should adjust my interpretations because obviously, I am wrong.

A car was initially traveling at an average speed of 48 miles per hour. During each of ten successive 10-minute intervals, the car decreased its average speed by 4 miles per hour. How many miles did the car travel in the seventh 10-minute interval?
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[spoiler]4

My answer was 3 1/3, which I just rounded down to 3. I started at 48, and by the time I got to the 7th interval, my speed was at 20 mph. Kaplan states that the first interval of decrease is at the 2nd interval. So the car was still moving at 48 during the first interval, and thus by the 7th interval was at 24 mph. Why should we assume that the first decrease was at the second interval instead of the first?[/spoiler]
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Re: Reading Questions Incorrectly.... Why? Need advice...

by ufdan » Tue Mar 10, 2009 1:53 pm
mkbigmoz wrote:I will post a question which I missed from the Kaplan CATs. I will also post my explanation on how I interpreted the question. Im looking for advice on how I should adjust my interpretations because obviously, I am wrong.

A car was initially traveling at an average speed of 48 miles per hour. During each of ten successive 10-minute intervals, the car decreased its average speed by 4 miles per hour. How many miles did the car travel in the seventh 10-minute interval?
2
3
4
5
6

[spoiler]4

My answer was 3 1/3, which I just rounded down to 3. I started at 48, and by the time I got to the 7th interval, my speed was at 20 mph. Kaplan states that the first interval of decrease is at the 2nd interval. So the car was still moving at 48 during the first interval, and thus by the 7th interval was at 24 mph. Why should we assume that the first decrease was at the second interval instead of the first?[/spoiler]
It says it initially starts at 48 and decrease by 4mph every ten minutes.

One thing that I've learned while studying is that if a problem is looking for a specific answer, like in this case, and you come up with an answer that is not a choice, you've done something wrong. Don't round up or down or approximate unless asked to do so.

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by hijazim » Wed Mar 11, 2009 6:26 am
In fact the way your thinking is right, but as you said in GMAT we shouldn't consider approximation unless asked to. So, we should consider another way of solving which i think don't contradict the given and leads to the right answer mentioned among the 5 choices.

Consider the whole time the car traveled = 10 X 10 mins = 100 mins

During 1st 10mins, d= 48x10 / 60 = 8 miles
During 2nd 10mins, d=44x10/60 = 7 1/3 miles
During 3rd 10mins, d=40x10/60 = 6 2/3
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During 7th 10mins, d=24x10/60= 4 miles

So, the right answer is C.

Hope it is clear now
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by Ian Stewart » Wed Mar 11, 2009 3:28 pm
That's a badly written question, so I'd just move on. For one thing, the question doesn't mention how the speed decreases - linearly, instantaneously, etc - which is clearly important here. If it is instantaneous, when does the change occur? That isn't specified either - it just says it happens 'during' each interval. If we're meant to assume the car was driving at 48 mph for the duration of the first ten minute interval, that's not at all clear from the question. That said, I imagine hijazim's solution above is the one intended by the question designer, but there's no way to be sure based on the wording of the question.

While questions from other sources are often badly worded, you do want to absolutely sure you understand the wording in any official questions - those you find in GMATPrep, GMATFocus, or the Official Guide. The questions you see on test day will have similar wording, and of course it's difficult to answer a question unless you understand it! GMAT questions are extensively edited and tested, and while the language can sometimes be abstract or confusing, as math can sometimes be, the questions are always precisely worded and unambiguous.

And ufdan's post above is excellent advice. Unless you're asked to approximate, if your answer doesn't match an answer choice, you either haven't simplified it the way the GMAT expects you to, or you've made a mistake. If you've made a mistake, there's no reason to think your answer should be close to the right answer.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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