Gmat Prep?? (Arithmetic Mean)

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Gmat Prep?? (Arithmetic Mean)

by dferm » Fri Aug 29, 2008 5:24 am
Please Help...
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by California4jx » Fri Aug 29, 2008 12:06 pm
B seems to be the right answer. Is it not, or are you asking for explanation?

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by dferm » Fri Aug 29, 2008 12:18 pm
the OA is C.... but don't quite know why....

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by parallel_chase » Fri Aug 29, 2008 6:00 pm
The answer is indeed C.

The two statements alone are clearly insufficient.

In first statement we dont know the value of m and in second statement we dont know the range of the set.

Combining I & II

m is odd positive integer

median of the set is 33

Set consists of consecutive integers multiples of 3.

RULE: In an evenly spaced set median=mean.

We know that the set is evenly spaced i.e. set of consecutive integers.
We know that median is 33

Hence mean or average = 33.

C is the answer.

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by lunarpower » Fri Aug 29, 2008 11:34 pm
the explanation given above by parallel_chase is fantastic.

in any case, the thing you have to look out for is this: DON'T MAKE UNWARRANTED ASSUMPTIONS.

in this problem, you must guard against the unwarranted assumption that the set of numbers is symmetrically distributed about its median / average. this assumption is reinforced by statement (1), which really does mean that the set must be symmetrically distributed, but statement (2) implies no such thing.

statement (2) talks only about the median of the set. you could have a terribly unbalanced set - say, 29, 31, 33, 35, 10982356217896452378 - with this median, and the mean would be a wildly different number.

moral of the story:
be sure to dissociate CONCEPTS from the PROBLEMS in which they appear.
for all those of you who thought the answer to this problem was (b), you probably fell prey to that trap because you are used to seeing "median" in problems that really do involve symmetrically distributed sets. if you've seen 3-4 median problems in a row in which you're told, as a precondition of the problem, that the set is symmetrically distributed (or are given a fact clearly implying that it is), it's easy to start assuming that that's always the case.
don't.
Ron has been teaching various standardized tests for 20 years.

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