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Help!

by kumard24 » Mon Mar 31, 2008 8:29 am
1. The 180 students in a group are to be seated in rows so that there is an equal number of students in each row. Each of the following could be the number of rows EXCEPT
(A) 4
(B) 20
(C) 30
(D) 40
(E) 90

the answer is D but dont understand why?
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Re: Help!

by AleksandrM » Mon Mar 31, 2008 9:32 am
kumard24 wrote:1. The 180 students in a group are to be seated in rows so that there is an equal number of students in each row. Each of the following could be the number of rows EXCEPT
(A) 4
(B) 20
(C) 30
(D) 40
(E) 90

the answer is D but dont understand why?
The answer is asking you which of the following answer choices cannot be evenly divided into 180.

You can right away eliminate E because 90 x 2 = 180

You can eliminate C because 30 x 6 = 180

You can eliminate B because 20 x 9 = 180

You can eliminate A (you can use long division if you like) because you get
45 x 4 = 180

SO, you are left with 40 if you cancel the zeros and divide 18 by 4, you use long division for this you will end up with 4.5, which is not an integer, that is your exception.
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by freedsl » Mon Mar 31, 2008 9:32 am
write out the prime factors of 180 = 2^2*5*3^2, the only number that you can't form with it is 40.

the only number that does not devide into 180 is 40, which is another way to look at the problem. 180?40 = 4.5 rows!
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by AleksandrM » Mon Mar 31, 2008 9:36 am
That's true, using prime factors is actually a much better strategy. I used my strategy for this problem because it is such an easy number to work with and saved me a lot of time. However, for more complex numbers, where it is not as evident, it is much MUCH better to use the factor foundation rule.
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by Stuart@KaplanGMAT » Mon Mar 31, 2008 9:42 am
AleksandrM wrote:That's true, using prime factors is actually a much better strategy.
I disagree!

Even though it's important to be good at concepts, and to understand those concepts, as we approach individual questions we also want to ask ourselves "what's the quickest way to answer this question?"

On this particular question, there's no doubt that brute force is the way to go. It's going to be MUCH faster to work with the answer choices and see which one doesn't work. We should start with the bigger choices, since they're easier to compare to 180. 90 goes in, eliminate (e). 40 doesn't go in: choose (d) - absolutely no need to check the remaining answers.

Remember, the GMAT is not a grade 9 math test in which questions are worth 20 points each, 1 for the right answer and 19 for showing your work. We need to become experts at zooming to the correct answers as quickly as possible and often traditional alegbraic approaches are MUCH slower than alternative solutions.
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by kumard24 » Tue Apr 01, 2008 9:24 am
Thanks again.

I have two weeks till my gmat. If i am getting sttuck on such easy questions then i think this is a bad sign. I think i will cancel the exam and study a little more.

Ciao for now
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