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# Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

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Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 04:16
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75% (01:37) correct 25% (01:49) wrong based on 442 sessions

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Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.

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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 20:43
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3

$$f(k) = \frac{k}{k+1}$$

$$f(f(k)) = \frac{\frac{k}{k+1}}{\frac{k}{k+1} + 1} = \frac{2}{3}$$

$$\frac{k}{k+1} * \frac{k+1}{2k+1} = \frac{2}{3}$$

3k = 4k + 2

k = -2
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Manager
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 04:41
2
1
Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.

F(f(k)) = 2/3 can be interpreted as follow:
f(x/(x+1)) = 2/3
i.e. (x/(x+1)) / (x/(x+1)) + 1 = 2/3

x/(2x+1) = 2/3
3x = 4x + 2
x = - 2

Thanks!
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 07:38
3
Given f(x) = x/(x + 1)

so f(k)=k/k+1

f(f(k))=f(k/k+1)=f[(k/k+1)/ (k/k+1)+1)]

Given f(f(k))=2/3

f(k/k+1)=f[(k/k+1)/ (k/k+1)+1)]=2/3
By solving , we will get

k=-2

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Senior Manager
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Posts: 275
Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 09:45
2
Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.

let's solve this problem by plugin approach.

let k=-2, then f(f(k))= f(f(-2))

f(-2) = -2/(-2+1) = 2
thus f(f(-2))= f(2)
f(2) = 2/(2+1) = 2/3

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Joined: 27 Nov 2014
Posts: 47
Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 10:25
2
Given f(x) =$$\frac{x}{(x + 1)}$$, for what value k does f(f(k)) = $$\frac{2}{3}$$ ?

A. –2
B.$$\frac{5}{3}$$
C. 1
D. 2
E. 8

Sol :

f(x) = $$\frac{x}{(x + 1)}$$
and
f(f(x) = $$\frac{x}{2x+1}$$........ 1

=> f(f(x) = $$\frac{2}{3}$$ ..........2

equating 1 and 2

$$\frac{x}{2x+1}$$ = $$\frac{2}{3}$$

Upon solving x = -2 hence ans A

Regards
SG
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 10:49
2
Starting with inner brackets :

f(f(k)) can be written as f(k/k+1)

Now, again applying the given condition on f(k/k+1)

So, f(k/k+1)= k/k+1/k/k+1 + 1 = 2/3

Or,k/2k+1 = 2/3

Or, k = -2

Ans : A
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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19 Jan 2015, 21:12
1
I too got A, -2 but with slightly different (and possibly convoluted) manner.

Lets say f(k) = y = x/(x+1). Solving backwards gives x=y/(1-y)

So if f(f(k)) = 2/3
f(k) = 2/3/(1-2/3) = 2.
k = 2/(1-2) = -2.

Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.
Manager
Joined: 26 Feb 2015
Posts: 115
Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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07 Mar 2015, 13:03
PareshGmat wrote:

$$f(k) = \frac{k}{k+1}$$

$$f(f(k)) = \frac{\frac{k}{k+1}}{\frac{k}{k+1} + 1} = \frac{2}{3}$$

$$\frac{k}{k+1} * \frac{k+1}{2k+1} = \frac{2}{3}$$

3k = 4k + 2

k = -2

What is the general formula for finding f(k) through f(f(k))? I don't understand that process.
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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07 Mar 2015, 16:23
1
1
Hi erikvm,

In Function questions such as this, you're given the formula that you need to work with. As such, you have to think more about how the math "works" within the given formula.

f(X) = X/(X+1)

Next, let's run through a few quick examples of how this function "works":

IF....X = 1
f(1) = 1/(1+1) = 1/2

IF....X = 2
f(2) = 2/(2+1) = 2/3

IF....X = 3
f(3) = 3/(3+1) = 3/4

These examples are pretty simple, but they help to establish the math that you will eventually have to do to answer this question.

Now, let's make it a bit more difficult....

What if we take the f(f(2))? This means that we have to use the function TWICE - the first time with the number 2, the second time with the RESULT of the first calculation...

f(2) = 2/(2+1) = 2/3

Since 2/3 is the result of the first calculation, we have to plug THAT value into the function to complete the second calculation....

f(2/3) = (2/3)/(2/3 + 1) = (2/3)/(5/3) = 2/5

So, the f(f(2)) = 2/5.

You should notice that the answers to this prompt are all numbers, so one of them MUST lead to the result (2/3) when you plug it into f(f(X)) = 2/3. Are you comfortable plugging in those answers and doing the math as outlined in the above example?

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Manager
Joined: 26 Feb 2015
Posts: 115
Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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08 Mar 2015, 03:57
EMPOWERgmatRichC wrote:
Hi erikvm,

In Function questions such as this, you're given the formula that you need to work with. As such, you have to think more about how the math "works" within the given formula.

f(X) = X/(X+1)

Next, let's run through a few quick examples of how this function "works":

IF....X = 1
f(1) = 1/(1+1) = 1/2

IF....X = 2
f(2) = 2/(2+1) = 2/3

IF....X = 3
f(3) = 3/(3+1) = 3/4

These examples are pretty simple, but they help to establish the math that you will eventually have to do to answer this question.

Now, let's make it a bit more difficult....

What if we take the f(f(2))? This means that we have to use the function TWICE - the first time with the number 2, the second time with the RESULT of the first calculation...

f(2) = 2/(2+1) = 2/3

Since 2/3 is the result of the first calculation, we have to plug THAT value into the function to complete the second calculation....

f(2/3) = (2/3)/(2/3 + 1) = (2/3)/(5/3) = 2/5

So, the f(f(2)) = 2/5.

You should notice that the answers to this prompt are all numbers, so one of them MUST lead to the result (2/3) when you plug it into f(f(X)) = 2/3. Are you comfortable plugging in those answers and doing the math as outlined in the above example?

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

Thank you for this explaination. I do understand the process of plugging in a value "twice", so to speak. However, now I don't have a value to plug in. I simply know what its gonna equate. How do I know that f(f(k)) can be written as f(k/k+1) ?
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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17 Apr 2015, 04:16
Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.

f(f(x) = (x/x+1)/((x/x+1)+1) = x/(2x+1)
so
x/(2x+1)=2/3
x=-2
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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30 Apr 2016, 23:28
Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

Kudos for a correct solution.

f(k)= k/(k+1)
f(f(k))= (k/(k+1)) / ( (k/(k+1)) +1 )
= (k/(k+1))/ ((2k+1)/(k+1))
= k / (2k+1)
so, k / (2k+1) =2/3
-> k= -2
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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?  [#permalink]

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04 Jul 2018, 18:30
Bunuel wrote:
Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ?

A. –2
B. 5/3
C. 1
D. 2
E. 8

f(k) = k/(k + 1)

f(k/(k + 1)) = k/(k + 1)/[k/(k + 1) + 1], thus:

k/(k + 1)/[k/(k + 1) + 1] = 2/3

We see that since the denominators on both sides of the equation are equal to one more than their respective numerators, so

k/(k + 1) = 2

k = 2(k + 1)

k = 2k + 2

-2 = k

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Re: Given f(x) = x/(x + 1), for what value k does f(f(k)) = 2/3 ? &nbs [#permalink] 04 Jul 2018, 18:30
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