ps - algebra

GMATGuruNY wrote:Below is the full question, which is from GMATPrep. If you don't want to be exposed to the question, read no further.

For all positive integers, [M]=3m when m is odd, [M]=1/2m when m is even. Which of the following is equivalent to [9]*[6]?

a) [81]
b) [54]
c) [37]
d) [27]
e) [18]

[M] = 3M when M is odd:
Thus, [9] = 3*9 = 27.

[M] is 1/2M when M is even:
Thus, [6] = 1/2*6 = 3.

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

Answer choice D:
Since [M] = 3M when M is odd, [27] = 3*27 = 81.

The correct answer is D.

yep.

the principal purpose of this problem, of course, doesn't have much to do with mathematical skill (the mathematics required in this problem is just elementary arithmetic). instead, the point of the problem is just to make sure that you are paying attention!
if you are being careless -- i.e., not noticing that the numbers in the answer choices are also enclosed in the same function brackets -- you'll pick choice (a), because the answer to the question is 81.

so, this question is really "careless = (a), careful = (d)". (i can't really see any mechanism by which somebody would get one of the other answer choices, other than random guessing.)