roc101 wrote:[m] = 3m for all odd m; [m] = 1/2m for all even m. solve [9] X [6]?

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

You are correct in that [9] x [6] = 81

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

However, the answer choices are as follows:

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

So, which of these equals 81?

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

So, the correct answer is D

Cheers,
Brent

Hi roc101,

Brent has correctly explained the math behind this question, so I won't rehash that material here.

This is an example of a "symbolism" question, in which the prompt "makes up" a math symbol, tells you what it means, and asks you to perform a basic calculation with it. You'll likely see 1 of these on the Official GMAT and you appeared to handle the bulk of the work just fine.

You ARE expected to use the symbol wherever it appears, so realizing the symbol appears in the answer choices means that you had a bit more work to do. This "twist" on a symbolism question isn't too common, but as you do better and better in the Quant section, the GMAT will adapt to you and give you more questions that require slightly more work (or that have "twists"). Be on the lookout for those details as you continue to study.

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

The temperature of a certain cup of coffee 10 minutes after it was poured was 120 degrees Fahrenheit. if the temperature f of the coffee t minutes after it was poured can be determined by the formula f = 120 * 2^(-at) + 60, where f is in degrees Fahrenheit and a is a constant, then the temperature of the coffee 30 minutes after it was poured was how many degrees Fahrenheit?
A. 65
B. 75
C. 80
D. 85
E. 90

The temperature of a certain cup of coffee 10 minutes after it was poured was 120 degrees Fahrenheit.

So, 120 = 120 * 2^[(-a)(10)] + 60
Divide both sides by 60: 2 = 2 * 2^[(-a)(10)] + 1
1 = 2 * 2^[(-a)(10)]
1/2 = 2^[(-a)(10)]
Since 2^(-1) = 1/2, we know that (-a)(10) = -1
So, a = 1/10

So, the formula is f = 120 * 2^[(-1/10)(t)] + 60

The temperature of the coffee 30 minutes after it was poured was how many degrees Fahrenheit?
f = 120 * 2^[(-1/10)(30)] + 60
= 120 * 2^[-3] + 60
= 120 * (1/8) + 60
= 15 + 60
= 75
= B

Cheers,
Brent

Hi roc101,

For future questions, you should plan to post each individually (in its own thread). You'll likely receive more responses and perspective from the experts and users in this Forum.

This is an example of a Work Formula question. Any time you have two entities (people, machines, water pumps, etc.) working on a job together, you can use the following formula:

(AB)/(A+B) = Total time to do the job together

Here, we're told that the total time = 4 hours and that one machine's rate is 1.5 times the other machine's rate...

If B = 1.5A then we have...

(A)(1.5A)/(A + 1.5A) = 4

1.5A^2/2.5A = 4

1.5A^2 = 10A

A^2 = 20A/3

A = 20/3 hours to fill the pool alone

With this info, we can plug back in and figure out the rate of the other pump (B = 10, so it would take 10 hours to fill the pool alone)

Final Answer: E

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