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How many factors of 3600 are even?

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How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 06:11
00:00
A
B
C
D
E

Difficulty:

  55% (hard)

Question Stats:

55% (01:27) correct 45% (01:10) wrong based on 116 sessions

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How many factors of 3600 are even?

A) 9
B) 12
C) 24
D) 36
E) 45

Source: http://www.GMATinsight.com

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Re: How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 08:18
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GMATinsight wrote:
How many factors of 3600 are even?

A) 9
B) 12
C) 24
D) 36
E) 45

Source: http://www.GMATinsight.com


\(3600=2^4*3^2*5^2\)...

Best way is to find ODD factors and then subtract from total..
Total = \((4+1)(2+1)(2+1)=5*3*3=45\)
Odd factors = \((2+1)(2+1)=3*3=9\)

So rest are even factors = 45-9=36

D

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Re: How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 06:45
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GMATinsight wrote:
How many factors of 3600 are even?

A) 9
B) 12
C) 24
D) 36
E) 45

Source: http://www.GMATinsight.com


3600 = 100 * 36 = 4 * 25 * 6 * 6 = 2^2 * 5^2 * 2^2 * 3^2 = 2^4 * 3^2 * 5^2

Find the total factors - odd factors = even factors

To find factors of a number we find the prime numbers = a^p * b^q...

The number of factors will be = (p+1) * (q+1)...

To find the number of odd factors we take all the other prime factors other than 2 in a similar fashion to the above

We take the powers of odd primes = (2+1)*(2+1) = 9

Total = (4 + 1) (2+1) (2+1) = 5 * 3 * 3 = 45

45 - 9 = 36

Answer choice D

I hope this helps!

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Re: How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 07:13
GMATinsight wrote:
How many factors of 3600 are even?

A) 9
B) 12
C) 24
D) 36
E) 45

Source: http://www.GMATinsight.com


Prime factorise the number 3600 i.e. \(3600 = 2^4*3^2*5^2\)
Row 1: 2^0, \(2^1, 2^2, 2^3, 2^4\) are factors of the number (except \(2^0\) because for number to be even every factor must have a minimum 21)
Row 2: \(3^0, 3^1, 3^2\) are factors of the number
Row 3: \(5^0, 5^1, 5^2\) are factors of the number

Total number of factors = (4)*(3)*(3) = 36 Factors
Answer: Option D
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How many factors of 3600 are even?  [#permalink]

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New post Updated on: 04 Nov 2018, 05:36
First make prime factorization of 3600=2^4 * 3^2 * 5^2 where 2, 3, and 5 are prime factors of 4, 2 and 2 are their powers.

The number of factors of 3600 will be expressed by the formula (p+1)(q+1)(r+1) where p,q and r are the powers.

According to the above the number of factors is (4+1)(2+1)(2+1)=45 factors...............................(1)

Now, get rid of powers of 2 as they give even factors, so the remaining left out numbers would be (2+1)(2+1)=9 factors......................(2)

All the remaining factors will be odd, therefore 3600 has 45-9=36 even factors.

Hence, the answer is D.


Hope it helps. :thumbup: if you like.

Originally posted by topper97 on 19 Oct 2018, 08:20.
Last edited by topper97 on 04 Nov 2018, 05:36, edited 1 time in total.
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Re: How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 08:40
Hi GMATinsight
Why cant we count normally ? Like 2^0(discarded)
2^1* any factor of 3/5 =1*6
2^2* any factor of 3/5 =1*6
2^3* any factor of 3/5 =1*6
2^4* any factor of 3/5 =1*6

total 24
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Re: How many factors of 3600 are even?  [#permalink]

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New post 19 Oct 2018, 08:58
ShankSouljaBoi wrote:
Hi GMATinsight
Why cant we count normally ? Like 2^0(discarded)
2^1* any factor of 3/5 =1*6
2^2* any factor of 3/5 =1*6
2^3* any factor of 3/5 =1*6
2^4* any factor of 3/5 =1*6

total 24


ShankSouljaBoi
What you are counting as 6 is actually 9 (Combination of all powers of 3 with all powers of 5)

2^1* any factor of 3/5 =1*9
2^2* any factor of 3/5 =1*9
2^3* any factor of 3/5 =1*9
2^4* any factor of 3/5 =1*9

Total = 36

I hope this helps!!!
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Re: How many factors of 3600 are even?  [#permalink]

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New post 05 Nov 2018, 22:36
Hi,

this was very helpful. Had a doubt though..so is there any way to directly figure the no of even factors? my question being why did we find the odd factors first and then substract from the total instead of directly finding the even?
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Re: How many factors of 3600 are even?  [#permalink]

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New post 06 Nov 2018, 01:24
Please scroll above. I have illustrated the same yashna36

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Re: How many factors of 3600 are even?  [#permalink]

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New post 06 Nov 2018, 06:08
yashna36 wrote:
Hi,

this was very helpful. Had a doubt though..so is there any way to directly figure the no of even factors? my question being why did we find the odd factors first and then substract from the total instead of directly finding the even?



Your doubt is valid and one solution is posted above but think from this perspective

Who would think that calculating even factors will be more difficult than calculating even factors???

because a layman idea would be to consider calculation of both equally easy/tough therefore it's always good to know both methods.

I hope this helps!!!
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Re: How many factors of 3600 are even? &nbs [#permalink] 06 Nov 2018, 06:08
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