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The function f(a) is defined for all positive integers a as the number

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The function f(a) is defined for all positive integers a as the number of positive integers which are less than a and have no common factor with a other than 1. What is f(77)?

A. 60
B. 63
C. 66
D. 70
E. 76
[Reveal] Spoiler: OA

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Re: The function f(a) is defined for all positive integers a as the number [#permalink]

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New post 24 Apr 2017, 05:46
f(77) contains numbers less than 77, which have no common factors with 77 other than 1.
The multiples of 7 and 11 will contain factors which are common.
The number 7 contains 10 such numbers from 7 till 70
Similarly the number 11 contains 6 numbers from 11 to 66
Hence f(77) = 76 - 10 - 6 = 60(Option A)
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Re: The function f(a) is defined for all positive integers a as the number [#permalink]

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Bunuel wrote:
The function f(a) is defined for all positive integers a as the number of positive integers which are less than a and have no common factor with a other than 1. What is f(77)?

A. 60
B. 63
C. 66
D. 70
E. 76


Nice question!

Since 77 = (11)(7), any positive multiple of 7 or 11 will have a common factor with 77 where that common factor is greater than 1.
For example, 77 and 14 share a common factor of 7.
And 77 and 35 share a common factor of 7.
Likewise, 77 and 55 share a common factor of 11.
And 77 and 33 share a common factor of 11.

Conversely, 77 and 10 do NOT share a common factor (other than a common factor of 1).
And 77 and 19 do NOT share a common factor (other than a common factor of 1).


There are 76 positive integers that are LESS THAN 77
The multiples of 7 that are less than 77 are as follows: 7, 14, 21, 28, . . . . 63, 70 (TOTAL = 10)
The multiples of 11 that are less than 77 are as follows: 11, 22, 33, . . . 55, 66 (TOTAL = 6)

So, f(77) = 76 - (10 + 6)
= 76 - 16
= 60

Answer:
[Reveal] Spoiler:
A


Cheers,
Brent
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Re: The function f(a) is defined for all positive integers a as the number [#permalink]

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New post 24 Apr 2017, 13:42
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Factors of 77=1, 7, 11, 77
+ive integers, which are <77 = 76
Exclusion from 'a' = 7, 14,..., 70 (10 numbers) + 11, 22,...,66 (6 numbers)
Answer: f(a) = 76-10-6 = 60
Choice: A

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Re: The function f(a) is defined for all positive integers a as the number [#permalink]

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New post 27 Apr 2017, 17:01
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Bunuel wrote:
The function f(a) is defined for all positive integers a as the number of positive integers which are less than a and have no common factor with a other than 1. What is f(77)?

A. 60
B. 63
C. 66
D. 70
E. 76


We are given that the function f(a) is defined for all positive integers a as the number of positive integers that are less than a and have no common factor with a other than 1, and we need to determine f(77).

We can prime factor 77 as 7 x 11. So now we can express the question as: How many positive integers less than 77 do not have 7 or 11 as factors? Our first step is to determine the number of multiples of 7 and 11 that are less than 77.

For 11: 11, 22, 33, 44, 55, 66 = 6 multiples

For 7: 7, 14, 21, 28, 35, 42, 49, 56, 63, 70 = 10 multiples

We can see that there are 6 + 10 = 16 integers that have common factors (other than 1) with 77. Since there are 76 positive integers less than 77 and 16 of them have common factors (other than 1) with 77, there are 76 - 16 = 60 numbers that have no common factors (other than 1) with 77.

Answer: A
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Re: The function f(a) is defined for all positive integers a as the number   [#permalink] 27 Apr 2017, 17:01
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