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# If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18

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Joined: 02 Sep 2009
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If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18  [#permalink]

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23 May 2018, 07:43
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Difficulty:

45% (medium)

Question Stats:

58% (01:35) correct 42% (01:45) wrong based on 104 sessions

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If $$f(g(3)) = 12$$, what is $$f(g(4))$$?

(1) $$g(x) = 2x + 2$$

(2) $$f(g(5)) = 18$$

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If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18  [#permalink]

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23 May 2018, 10:37
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Bunuel wrote:
If $$f(g(3)) = 12$$, what is $$f(g(4))$$?

(1) $$g(x) = 2x + 2$$

(2) $$f(g(5)) = 18$$

To get the value of $$f(g(4))$$ we need the definition of both the function $$f(x)$$ & $$g(x)$$

Statement 1: this defines $$g(x)$$ but nothing mentioned about $$f(x)$$. Insufficient

Statement 2: this does not define any function. it simply provide the value of the function for a certain constant. Insufficient

Combining 1 & 2: definition of function $$f(x)$$ is unavailable. Hence $$f(g(4))$$ cannot be found out. Insufficient

Option E
Manager
Joined: 23 Nov 2017
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Location: India
GMAT 1: 720 Q51 V36
Re: If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18  [#permalink]

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24 May 2018, 04:29
Bunuel wrote:
If $$f(g(3)) = 12$$, what is $$f(g(4))$$?

(1) $$g(x) = 2x + 2$$

(2) $$f(g(5)) = 18$$

The data is sufficient if we can find a unique value for f(g(4)).
From question stem we know f(g(3)) = 12.

Statement 1:$$g(x) = 2x + 2$$
We will be able to compute g(3)to be 8 and g(4) to be 10

f(g(3)) = f(8) = 12.
f(g(4)) = f(10) = ?

It coule be any value depending on what f(x) is. Here are two such values it can take.
If f(x) = 1.5x, f(8) = 12 and f(10) = 15
Alternatively, if f(x) = x + 4, f(8) = 12 and f(10) = 14.

Without additional information about f(x), we cannot find a unique value for f(g(4))

Statement 1 alone is NOT sufficient.

Statement 2: $$f(g(5)) = 18$$
Without knowing what g(x) is and what f(x), we will not be able extrapolate with this information and the question stem.
For instance, if f(g(x)) = 3x + 3, then f(g(3)) = 12 and f(g(5)) = 18. And f(g(4)) = 15
Alternatively, if f(g(x)) = sqrt(1.5) * f(g(x - 1)), f(g(5)) = sqrt(1.5) * f(g(4)) = sqrt(1.5) * sqrt(1.5) f(g(3)) = 1.5 * 12 = 18 and f(g(4)) = sqrt(1.5) * 12 which is not 15.

Statement 2 alone is not sufficient.

Statements together: g(x) = 2x + 2 and f(g(5)) = 18
g(3) = 8, g(4) = 10 and g(5) = 12
For instance, f(x) = 1.5x. If so, f(g(3)) = f(8) =12; f(g(5)) = f(12) = 18 and f(g(4)) = f(10) = 15
Alternatively, f(x) = sqrt (1.5)f(x - 2). If so, f(g(5)) = f(12) = sqrt(1.5)*f(10) = sqrt(1.5) * sqrt(1.5) * f(8) = 1.5 * 12 = 18
And f(g(4)) = f(10) = sqrt(1.5) * f(8) = sqrt(1.5) * 12 which is not 15.

Statements together not sufficient. Choice E is the answer.
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Joined: 30 Mar 2017
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GMAT 1: 200 Q1 V1
Re: If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18  [#permalink]

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18 Jul 2018, 16:46
No work necessary. We don't know f(x) so answer is E.
Re: If f(g(3)) = 12, what is f(g(4))? (1) g(x) = 2x + 2 (2) f(g(5)) = 18 &nbs [#permalink] 18 Jul 2018, 16:46
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