GMAT Prep question

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sathishkumarjva9888: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Sun Dec 18, 2011 7:11 pm; Thanked: 1 times

by sathishkumarjva9888 » Sun Apr 29, 2012 8:10 am

For every positive integer m, m = 3m , when m is odd and m= m/2, when m is even. What is the value of 9*6?

A.) 81
B.) 54
C.) 36
D.) 27
E.) 18

OA: C

Can someone clarify why A is wrong?

NOTE: There was a symbol around m in the Prep s/w. Since i couldnt post the symbol, have modified it to italics.

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Source: Beat The GMAT — Problem Solving | Sun Apr 29, 2012 8:10 am

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Apr 29, 2012 9:58 am

sathishkumarjva9888 wrote:For every positive integer m, m = 3m , when m is odd and m= m/2, when m is even. What is the value of 9*6?

A.) 81
B.) 54
C.) 36
D.) 27
E.) 18

OA: C

Can someone clarify why A is wrong?

NOTE: There was a symbol around m in the Prep s/w. Since i couldnt post the symbol, have modified it to italics.

For all positive integers m,[m] = 3m when m is odd and [m] = m/2 when m is even. Which of the following is equivalent to [9]*[6]?

Now [m] = 3m if m is odd, and [m] is m/2 if m is even.
[9] = 3 * 9 = 27
[6] = 6/2 = 3

So [9] * [6] = 27 * 3 = 81 = 3 * 27 = [27]

The correct answer is D.

Note that in the original question, the answer choices are also in squares, which means 81 = 3 * 27 = [27], according to the given symbols. So, you solved it right, but in the end you got trapped in choosing the correct answer. I hope you get my point.

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sathishkumarjva9888: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Sun Dec 18, 2011 7:11 pm; Thanked: 1 times

by sathishkumarjva9888 » Sun Apr 29, 2012 10:09 am

Thanks anurag.

But i still do not understand why should we again split 81 as 3*27.

Is it because the question was to find the equivalent of [9]*[6] and not the value of [9]*[6]??

I would like to know how to tackle these kind of traps. The question seemed really easy but never gave a thought that i should split up 81 again.

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Apr 29, 2012 10:14 am

sathishkumarjva9888 wrote:Thanks anurag.

But i still do not understand why should we again split 81 as 3*27.

Is it because the question was to find the equivalent of [9]*[6] and not the value of [9]*[6]??

I would like to know how to tackle these kind of traps. The question seemed really easy but never gave a thought that i should split up 81 again.

It is just because the answer choices are not given without square symbol, and in the question itself it is clearly mentioned that for an odd integer m, [m] = 3m
So, [27] = 3 * 27 = 81
Now if you look at the answer choices it is not simply 81, it is within a square, which means if you choose [81] as the correct answer then [81] = 3 * 81, which is not the case.

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sathishkumarjva9888: Senior | Next Rank: 100 Posts; Posts: 46; Joined: Sun Dec 18, 2011 7:11 pm; Thanked: 1 times

by sathishkumarjva9888 » Sun Apr 29, 2012 10:28 am

Thanks for pointing that Anurag.

So, i should see [9]*[6] = 81 as [27] and not as plain 81. I presume the word "equivalent" in the question stem is the key for me to look at each answer choice with [].

Correct me if i am wrong.

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Anurag@Gurome: GMAT Instructor; Posts: 3835; Joined: Fri Apr 02, 2010 10:00 pmFollowed by: 523 members; Location: Milpitas, CA; Thanked: 1854 times; GMAT Score:770

by Anurag@Gurome » Sun Apr 29, 2012 5:04 pm

sathishkumarjva9888 wrote:Thanks for pointing that Anurag.

So, i should see [9]*[6] = 81 as [27] and not as plain 81. I presume the word "equivalent" in the question stem is the key for me to look at each answer choice with [].

Correct me if i am wrong.

Yes, that's right.
[9] * [6] = 81 = [27]

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