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If abc = b^3 , which of the following must be true?

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If abc = b^3 , which of the following must be true? [#permalink]

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If abc = b^3 , which of the following must be true?

I. ac = b^2
II. b = 0
III. ac = 1

A. None
B. I only
C. II only
D. I and III
E. II and III

This is from Kaplan CAT. I am not quite sure how they arrived at the answer.
[Reveal] Spoiler: OA

Last edited by Bunuel on 30 Mar 2012, 10:44, edited 1 time in total.
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Re: If abc = b3 , which of the following must be true? [#permalink]

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abc = b^3 implies abc - b^3 = 0
or b*(ac-b^2)=0
which implies either b=0 or ac=b^2
so, either of them or both of them can be true, but none of them must be true. Answer A
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Re: If abc = b3 , which of the following must be true? [#permalink]

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New post 30 Mar 2012, 10:12
Hi Bunuel,
Request you to post your reasoning. Here is how I approached this one-
abc = b^3
abc-b^3 =0
b(ac-b^2)=0
Hence either b=0 or ac=b^2

However b=0 is not a solution. Could you please shed some light on how to approach must be true or could be true questions.
Thanks
H
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Re: If abc = b3 , which of the following must be true? [#permalink]

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New post 30 Mar 2012, 10:55
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imhimanshu wrote:
Hi Bunuel,
Request you to post your reasoning. Here is how I approached this one-
abc = b^3
abc-b^3 =0
b(ac-b^2)=0
Hence either b=0 or ac=b^2

However b=0 is not a solution. Could you please shed some light on how to approach must be true or could be true questions.
Thanks
H


If abc = b^3 , which of the following must be true?
I. ac = b^2
II. b = 0
III. ac = 1

A. None
B. I only
C. II only
D. I and III
E. II and III

\(abc = b^3\) --> \(b(ac-b^2)=0\) --> EITHER \(b=0\) OR \(ac=b^2\), which means that NONE of the option MUST be true.

For example if \(b=0\) then \(ac\) can equal to any number (not necessarily to 0 or 1), so I and III are not always true, and if \(ac=b^2\) then \(b\) can also equal to any number (not necessarily to 0), so II is not always true.

Answer: A.

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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 31 Mar 2012, 07:44
Thanks Bunuel.. you are awesum :-)
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Re: Number properties [#permalink]

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New post 06 May 2012, 18:34
OA is definitely A. Folks may be little bit confused at option *i and iii* (3) tells us that ac=1. So what? Is b also 1 or equal to 0? Can't figure it out. Move further. If some part of multiplication is 0, there is no need to ans. That question. It doesnt satisfy the condition: abc=b^3.

NOTE:WHEN YOU ARE TACKLING "MUST BE TYPE" QUESTION, always try to find out " entire set" instead of "sub set" cz subset answers "what should be" rather than "what must be".
If you are an accountant, you may consider it equivalent to " Bad debt reserve".

Entire set= LCM
Suset =GCM.

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Re: If abc = b^3 , which of the following must be true? [#permalink]

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nikhilsrl wrote:
If abc = b^3 , which of the following must be true?

I. ac = b^2
II. b = 0
III. ac = 1

A. None
B. I only
C. II only
D. I and III
E. II and III

This is from Kaplan CAT. I am not quite sure how they arrived at the answer.


Responding to a pm:

I think I won't be able to take 'Must be true' for another week on my blog (Std Dev still not over). Hence, explaining this question right here.

Given: \(abc = b^3\)
\(abc - b^3 = 0\)
\(b(ac - b^2) = 0\)

This tells us that either b = 0 OR \(ac = b^2\). At least one of these 2 'must be true'.

So can I say b must be equal to 0? No! It is possible but it is also possible that instead, \(ac = b^2\).
So can I say ac must be equal to \(b^2\)? No! It is possible but it is also possible that instead, b = 0.
So can I say that b = 0 AND \(ac = b^2\)? No! It is possible but it is also possible that only one of them is true.
So can I say that b = 0 OR \(ac = b^2\)? Yes, I can! One of them must be true.

If one of the options were 'I or II', that would have been true.
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Re: If abc = b^3 , which of the following must be true? [#permalink]

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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 10 Oct 2015, 20:57
Hello from the GMAT Club BumpBot!

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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 11 Mar 2016, 10:16
Why is it wrong to divide both sides by b at the start?
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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 11 Mar 2016, 10:17
Why is it wrong to divide both sides by b at the start?
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Re: If abc = b^3 , which of the following must be true? [#permalink]

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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 14 Mar 2016, 03:16
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STEMBusiness wrote:
Why is it wrong to divide both sides by b at the start?


Another way to understand this:

When you divide by b, you are assuming that b is not 0. Is it a fair assumption? No. You are losing on a possible solution of the equation.

How can I make abc = b^3? I can do that by either making b = 0 or making ac = b^2.

But if you divide by b, you get only one solution: ac = b^2 and that means I must be true. Actually, the answer is none because there is another possibility and that is b = 0.
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Re: If abc = b^3 , which of the following must be true? [#permalink]

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New post 15 Mar 2016, 00:51
nikhilsrl wrote:
If abc = b^3 , which of the following must be true?

I. ac = b^2
II. b = 0
III. ac = 1

A. None
B. I only
C. II only
D. I and III
E. II and III

This is from Kaplan CAT. I am not quite sure how they arrived at the answer.

HERE the question is asking us which statement must be true
Hence we can discard all of them as we can make each one of them untrue by taking the other leftover equations true.
Hence A
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Re: If abc = b^3 , which of the following must be true?   [#permalink] 15 Mar 2016, 00:51
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