For all positive integers m

[BTGmoderatorDC]

For all positive integers m, [m]=3m when m is odd and [m]=(1/2)*m when m is even. What is [9]*[6] equivalent to?

A. [81]
B. [54]
C. [37]
D. [27]
E. [18]

How will i solve this kind of problem?

OA D

[Brent@GMATPrepNow]

For all positive integers m, [m]=3m when m is odd, and [m]=(1/2)m when m is even. What is [9][6] equivalent to?

A. [81]
B. [54]
C. [37]
D. [27]
E. [18]

9 is odd, so [9] = (3)(9) = 27
6 is even, so [6] = 6/2 = 3
So, [9] x [6] = 27 x 3 = 81

BEFORE you select A, notice that 81 has brackets around it.
Since 81 is odd, [81] = (3)(81) = 243
So, answer choice A is NOT correct.

So, which of the 5 answer choices equals 81?

Since 27 is odd, [27] = (3)(27) = 81

So, the correct answer is D

Cheers,
Brent

[myspecialtie]

Hi Brent, thanks for your explanation. The brackets around the answers was not even something I was looking at when answering the question. Is it typical of GMAT questions to have little tricks like these for their solutions?

[[email protected]]

Hi myspecialtie,

Brent's explanation is spot-on, so I won't rehash any of that here. This prompt is an example of a "Symbolism" question - in which the prompt introduces a "made up" math symbol, explains how to use it and asks you to perform a calculation (and sometimes more than one) with it. These types of prompts are relatively rare (you'll likely see just one on Test Day) and are generally built around standard Arithmetic or Algebra, so the 'math' behind them isn't that difficult. You do need to pay careful attention to how the Symbol is used though (in this prompt, the Symbol is incorporated into each of the answer choices - and that is a rare 'twist' that you likely will NOT see on Test Day).

GMAT assassins aren't born, they're made,
Rich

[Brent@GMATPrepNow]

Brett's explanation is spot-on

Thanks Roach!

Cheers,
Brent - now with 50% less "t"!!

[[email protected]]

Correctted!

And yes - that typo was on purpose ;o.

[Scott@TargetTestPrep]

For all positive integers m, [m]=3m when m is odd and [m]=(1/2)*m when m is even. What is [9]*[6] equivalent to?

A. [81]
B. [54]
C. [37]
D. [27]
E. [18]

How will i solve this kind of problem?

OA D

We are given that [m] = 3m when m is odd, and [m] = (1/2)*m when m is even, and we must determine the value of [9]*[6].

Since 9 is odd, [9] = 3 x 9 = 27.

Since 6 is even, [6] = (1/2) x 6 = 3.

Thus, [9]*[6] = 27 x 3 = 81.

Now we must determine which "bracketed" answer choice is equal to 81.

Since 27 is odd, we see that [27] = 27 x 3 = 81.

Answer: D