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Math Expert V
Joined: 02 Sep 2009
Posts: 56244
For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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11 00:00

Difficulty:   25% (medium)

Question Stats: 72% (01:09) correct 28% (01:38) wrong based on 342 sessions

### HideShow timer Statistics For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7)?

A. 8
B. 12
C. 15
D. 21
E. 35

Kudos for a correct solution.

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Math Expert V
Joined: 02 Sep 2009
Posts: 56244
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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1
Bunuel wrote:
For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7)?

A. 8
B. 12
C. 15
D. 21
E. 35

Kudos for a correct solution.

VERITAS PREP OFFICIAL SOLUTION:

This problem really just comes down to understanding the function notation. If f(x)=f(x+1), that means that:

f(3)=f(3+1)=f(4)
and that

f(4)=f(4+1)=f(5)
and so on, meaning that really all functions equal the same thing as the next function up, so all functions are equal. That means that f(7) is the same thing as f(3), which we're told is 15. Therefore the answer is 15.
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Math Expert V
Joined: 02 Aug 2009
Posts: 7764
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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ans C... all values of x should be the same ie 15.. unless i am missing something here
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Manager  G
Joined: 13 Oct 2013
Posts: 136
Concentration: Strategy, Entrepreneurship
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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1
f(x)=f(x+1)
f(3)=15
=>f(3)=f(3+1)=f(4)=f(4+1)=f(5)=f(5+1)=f(6)=f(6+1)=f(7)=15

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Kindly press +1 Kudos if my post helped you in any way Originally posted by sunita123 on 27 Jan 2015, 16:05.
Last edited by sunita123 on 28 Jan 2015, 21:55, edited 1 time in total.
Manager  Joined: 27 Oct 2013
Posts: 203
Location: India
Concentration: General Management, Technology
GMAT Date: 03-02-2015
GPA: 3.88
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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3
Here we go---

f(x) = f(x+1) ---(1)

f(3) = 15 ----(2)

Now

f(7) = f(6+1)-----> f(6) = f(5+1)-------> f(5) = f(4+1) ------> f(4) = F(3+1)---> f(3) = 15

Now backtrack
f(4) = 15-----> f(5) = 15 ------> f(6) = 15 ----> f(7) = 15

Though I picked option C

I am still not 100% sure of the answer(don't know why)

My bets on C.
Manager  Joined: 15 Aug 2013
Posts: 53
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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1
It is given that: f(x) = f(x+1) implying,
for any value of x, f(x) and f(x +1) has same value => this function is a constant value function.
so for tis function, f(1) = f(2) = f(3) = .... = f(7) = ... f (n-1) = f(n) = 15.
Hence, Ans is (C).
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Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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f(3) = f(4) = f(5) = f(6) = f(7) = 15

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Kindly press "+1 Kudos" to appreciate Intern  Joined: 08 Jun 2011
Posts: 18
Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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f(x) = f(x+1) means,
for any x, f(x) and f(x +1) has same value
so f(1) = f(2) = f(3) = f(x) = f(x+1)= 15.
Hence, Ans is (C).
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Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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_________________ Re: For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7   [#permalink] 11 Jan 2019, 02:04
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