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

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For all positive integers x, f(x)=f(x+1). If f(3)=15, then what is f(7  [#permalink]

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27 Jan 2015, 07:49
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Question Stats:

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

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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

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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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02 Feb 2015, 03:00
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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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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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27 Jan 2015, 08:39
2
ans C... all values of x should be the same ie 15.. unless i am missing something here
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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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Updated on: 28 Jan 2015, 21:55
1
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

so Answer is C

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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.
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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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28 Jan 2015, 02:54
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.
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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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29 Jan 2015, 13:44
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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03 Feb 2015, 21:04
1
f(3) = f(4) = f(5) = f(6) = f(7) = 15

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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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02 Jul 2017, 10:07
1
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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11 Jan 2019, 02:04
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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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