Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 08 Feb 2016, 18:01

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

question from MGMAT test -

Author Message
TAGS:
Manager
Joined: 29 Jul 2009
Posts: 178
Followers: 4

Kudos [?]: 58 [0], given: 2

question from MGMAT test - [#permalink]  18 Apr 2010, 07:30
1
This post was
BOOKMARKED
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions
If #p# = ap3+ bp – 1 where a and b are constants, and #-5# = 3, what is the value of #5#?

The answer I found was (5) and here is how I solved the question.
for #-5# = 3, here is the equation -125a – 5b – 1 = 3
For #5#, the equation is 125a + 5b – 1 = X

then to find X, I substract the first equation from the second one

-125a – 5b – 1 = 3
-
125a + 5b – 1 = X

that gives -2 = 3-x, so x = 5

However, the correct answer is (-5) and here is how MGMAT solved it. I got their explanation, but I'm curious to know why my way didn't work!

Since we know the value of #-5# = 3, we can plug p = -5 into our formula:

(-5)3a + (-5)b – 1 = 3
-125a – 5b – 1 = 3
-125a – 5b = 4

We are asked to solve for #5#. If we plug 5 into our formula, we get:
(5)3a + (5)b – 1 = ?
125a + (5)b – 1 = ?

To figure this out, we would need to know the value of 125a + 5b.

From the first equation we know that -125a – 5b = 4. By multiplying both sides by negative one, we see that 125a + 5b = -4.

125a + 5b – 1 = ?
–4 – 1 = -5

 Manhattan GMAT Discount Codes Kaplan GMAT Prep Discount Codes Knewton GMAT Discount Codes
Math Expert
Joined: 02 Sep 2009
Posts: 31286
Followers: 5348

Kudos [?]: 62190 [0], given: 9444

Re: question from MGMAT test - [#permalink]  18 Apr 2010, 07:46
Expert's post
If #p# = ap3+ bp – 1 where a and b are constants, and #-5# = 3, what is the value of #5#?

The answer I found was (5) and here is how I solved the question.
for #-5# = 3, here is the equation -125a – 5b – 1 = 3
For #5#, the equation is 125a + 5b – 1 = X

then to find X, I substract the first equation from the second one

-125a – 5b – 1 = 3
-
125a + 5b – 1 = X

that gives -2 = 3-x, so x = 5

Your approach is much better than that of MGMAT's. Though there are two problems:

When you subtract equations you'll get: $$-250a-10b=3-x$$ (so you subtracted incorrectly) and this doesn't work.

What you should have done is to add equations instead of subtracting: $$-125a-5b-1+125a+5b-1=3+x$$ --> $$-2=3+x$$ --> $$x=-5$$.

Hope it helps.
_________________
Re: question from MGMAT test -   [#permalink] 18 Apr 2010, 07:46
Similar topics Replies Last post
Similar
Topics:
1 Question from PrepTest 1 3 18 Sep 2011, 08:08
MGMAT test 1 17 Jun 2011, 03:03
From MitDavidDv: Problem Solving Question- MGMAT CAT #2 2 11 Jun 2011, 21:16
2 Difficult questions from a practice test 2 30 Apr 2011, 19:45
Question from Gmat club test 1 08 Jul 2010, 03:57
Display posts from previous: Sort by