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Re: Data Sufficiency Challenge Problem [#permalink]
16 Nov 2010, 23:55

[quote="VeritasPrepBrian"]Hello, community:

Try out this problem that I just wrote up for one of my students:

What is the value of product abc?

1) 2^a * 3^b * 5^c = 1728

2) a, b, and c are nonnegative integers[/quote]

1728 = 2^6 * 3^3 * 5^0

(1) If a,b,c are not integers, they can take infinite values Eg (6,3,0) is te integral solution Take c=0, a=1 then b = [m]log_3(864)[/m] Take c=0, a=2 then b = [m]log_3(432)[/m]

(2) Not sufficient to know the product

(1+2) Since prime factorisation is unique, a=6,b=3,c=0 ()--> abc=0
_________________

Re: Data Sufficiency Challenge Problem [#permalink]
17 Nov 2010, 09:36

1

This post received KUDOS

Thanks for the responses, everyone! I love this question because of the strategy it brings up, which we call:

Why Are You Here?

Regarding statement 2, it's nowhere close to being sufficient on its own. So there are two likely reasons that it's there:

1) To trick you into thinking that you need it, and therefore picking C instead of A 2) To add information that IS, in fact, necessary to go with statement 1, so that the correct answer is C and not A

The GMAT doesn't use "red herring" statements - those that are simply so far out of scope that they're not even relevant - very often at all; if they provide a statement in a Data Sufficiency problem there has to be a reason...it's either a trap or it's necessary information. The good news for you is that you can use either case the same way: look at that statement to determine whether you really need it.

Here, although statement 1 may seem sufficient on its own (c must be 0 in order to make the 5 term equal to 1, since 1728 has no multiples of 5), that only fits if we know that they're all integers. Statement 2, by providing us with that information explicitly (they're nonnegative integers), should make us pause to think about statement 1: Do they have to be integers?

We don't need to use logarithms on the GMAT (thankfully!), but we should know enough that there would conceivably exist a set of noninteger exponents that would solve this problem. Even if you just assume that a and b are 1 so that:

5^c = 288

There is some value for c that will get us 288, so we can prove that statement 1 is not on its own sufficient. We need statement 2's help to determine that they're integers, so the correct answer is C.

So, strategically, when you see a statement that on its own is clearly not sufficient, ask yourself "why are you here?". Is it providing essential information, or is it there to make you think you need it?
_________________

Re: Data Sufficiency Challenge Problem [#permalink]
17 Nov 2010, 13:51

It should be C. (1) does not give me enough information. There are different variations of a,b,c (integers, non-integers, lagorithmus, positive and negative values) that will satisfy the equations. insufficient

(2) alone insufficient.

Combining (1) and (2) we have now the information that supplement statement (1). If we know that a,b,c are non-negative integers, we could then say that the only possible values are 6,3 and 0 (1728=2^6*3^3*5^0) and the value of a*b*c=0 Hence C

Re: Data Sufficiency Challenge Problem [#permalink]
17 Nov 2010, 23:47

A for me. 2^a * 3^b * 5^c = 1728 would mean that the factor of 5 does not contribute to the multiplication (mutiples of 5 end with 5 or 0). Hence the value of c = 0. By knowing this, I can say that the value of abc = 0.

Re: Data Sufficiency Challenge Problem [#permalink]
30 Apr 2014, 08:31

VeritasPrepBrian wrote:

Thanks for the responses, everyone! I love this question because of the strategy it brings up, which we call:

Why Are You Here?

Regarding statement 2, it's nowhere close to being sufficient on its own. So there are two likely reasons that it's there:

1) To trick you into thinking that you need it, and therefore picking C instead of A 2) To add information that IS, in fact, necessary to go with statement 1, so that the correct answer is C and not A

The GMAT doesn't use "red herring" statements - those that are simply so far out of scope that they're not even relevant - very often at all; if they provide a statement in a Data Sufficiency problem there has to be a reason...it's either a trap or it's necessary information. The good news for you is that you can use either case the same way: look at that statement to determine whether you really need it.

Here, although statement 1 may seem sufficient on its own (c must be 0 in order to make the 5 term equal to 1, since 1728 has no multiples of 5), that only fits if we know that they're all integers. Statement 2, by providing us with that information explicitly (they're nonnegative integers), should make us pause to think about statement 1: Do they have to be integers?

We don't need to use logarithms on the GMAT (thankfully!), but we should know enough that there would conceivably exist a set of noninteger exponents that would solve this problem. Even if you just assume that a and b are 1 so that:

5^c = 288

There is some value for c that will get us 288, so we can prove that statement 1 is not on its own sufficient. We need statement 2's help to determine that they're integers, so the correct answer is C.

So, strategically, when you see a statement that on its own is clearly not sufficient, ask yourself "why are you here?". Is it providing essential information, or is it there to make you think you need it?

Re: What is the value of product abc? [#permalink]
04 May 2014, 08:26

hi brian

so basically we should never assume that the numbers r integers if it is not given... also i need some explanation for statement one.... how is a,b,c being integers matter?... if they were fractions... how would they give us that number 1728?
_________________

Hope to clear it this time!! GMAT 1: 540 Preparing again

Re: What is the value of product abc? [#permalink]
20 May 2014, 06:25

NGGMAT wrote:

hi brian

so basically we should never assume that the numbers r integers if it is not given... also i need some explanation for statement one.... how is a,b,c being integers matter?... if they were fractions... how would they give us that number 1728?

When a, b, c are integers, and we have first statement as well, we need to put c=0 because 1728 has no 5 in it, and we have to make 5^c as 1. Even if they are fractions there can be two cases,

Case 1: They can be rational numbers/fractions, in which case the information coupled with statement 1 will be sufficient.

Case 2: They can be irrational numbers/fractions. In this case it won't suffice.

a, b and c can have values: 6, 3 and 0 respectively. or if we take values for a and b to be 5, 3, we get: 5^c = 2 =>c=\frac{log 2}{log5} if we take values for a and b to be 6, 2, we get: 5^c = 3 =>c= \frac{log 3}{log5}

Multiple solutions exist. So not sufficient

Statement 2: a, b, and c are nonnegative integers

Not sufficient

Combining both Statements,

we get, a = 6, b= 3 and c= 0

Hence, Answer is C _________________

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gmatclubot

Re: What is the value of product abc?
[#permalink]
23 May 2014, 02:04