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Magoosh GMAT Instructor
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Re: if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
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stonecold wrote:
if P represents the number of prime numbers between 1 and 102 then what is the value of P?
[A] 25
26
[C] 27
[D] 28
[E] 29


All prime numbers except 2 and 3 can be expressed as -

6n + 1

and

6n - 1


Find the value of n -

n can be 100/6 = 16

Now form a table -

Attachment:
Capture.PNG
Capture.PNG [ 7.95 KiB | Viewed 3942 times ]


So we have 23 numbers between 5 to 97 and 3 more numbers (2, 3 and 101 )

[b]Hence total we have 26 numbers in the given range... :-D :lol: :x
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if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
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Abhishek009 wrote:
stonecold wrote:
if P represents the number of prime numbers between 1 and 102 then what is the value of P?
[A] 25
26
[C] 27
[D] 28
[E] 29


All prime numbers except 2 and 3 can be expressed as -

6n + 1

and

6n - 1


Find the value of n -

n can be 100/6 = 16

Now form a table -

Attachment:
Capture.PNG


So we have 23 numbers between 5 to 97 and 3 more numbers (2, 3 and 101 )

[b]Hence total we have 26 numbers in the given range... :-D :lol: :x



Excellent Solution...!!
Unfortunately 5 kudos limit has been already surpassed.

StoneCold
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if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
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stonecold wrote:

Excellent Solution...!!
Unfortunately 5 kudos limit has been already surpassed.

StoneCold


A different approach without use of 6n+1 and 6n -1

Units digit of a prime number except 2 will always be odd ( 1,3 , 5 , 7 , 9 )

So start working around.

01 - 09 = 2, 3 ,5 , 7
10 - 19 = 11, 13 , 17 , 19
20 - 29 = 23 , 29
30 - 39 = 31 , 37
40 - 49 = 41 , 43 , 47
50 - 59 = 53 , 59
60 - 69 = 61 , 67
70 - 79 = 71 , 73 , 79
80 - 89 = 83 , 87 , 89
90 - 99 = 97
100 - 102 = 101

Count you will get 26 Prime numbers

[b]stonecold
no issues Austin Bro :-D :lol: , the satisfaction of helping someone > Kudos!! ( Expression in Mathematical form because I am posting in Quants Section ) :lol:


Would love to have someone with any alternative solution to the same question, indeed its a beautiful question.
Magoosh GMAT Instructor
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Re: if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
Expert Reply
stonecold wrote:
Can you please post a question based on odds are prime except the multiple of 5 are prime between 100 and 110, as is true between 10 and 20. This is also true between 190 and 200.
Sounds very interesting.
Do share the link if it already exists.
Thanks
Stone Cold

Dear Stone Cold,
My friend, I was inspired by this question and this discussion, and I posted a related question here:
how-many-integers-n-are-prime-numbers-in-the-range-200-n-217055.html
That one can be solved very systematically without a calculator.
Mike :-)
Alum
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Re: if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
Abhishek009 wrote:
stonecold wrote:

Excellent Solution...!!
Unfortunately 5 kudos limit has been already surpassed.

StoneCold


A different approach without use of 6n+1 and 6n -1

Units digit of a prime number except 2 will always be odd ( 1,3 , 5 , 7 , 9 )

So start working around.

01 - 09 = 2, 3 ,5 , 7
10 - 19 = 11, 13 , 17 , 19
20 - 29 = 23 , 29
30 - 39 = 31 , 37
40 - 49 = 41 , 43 , 47
50 - 59 = 53 , 59
60 - 69 = 61 , 67
70 - 79 = 71 , 73 , 79
80 - 89 = 83 , 87 , 89
90 - 99 = 97
100 - 102 = 101

Count you will get 26 Prime numbers

[b]stonecold
no issues Austin Bro :-D :lol: , the satisfaction of helping someone > Kudos!! ( Expression in Mathematical form because I am posting in Quants Section ) :lol:


Would love to have someone with any alternative solution to the same question, indeed its a beautiful question.



Kudos Visible ..!!
:)
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Re: if P represents the number of prime numbers between 1 and 102 then wha [#permalink]
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