Author 
Message 
TAGS:

Hide Tags

Manager
Joined: 07 Jan 2010
Posts: 103
Location: So. CA
WE 1: 2 IT
WE 2: 4 Software Analyst

How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
08 Sep 2010, 22:58
Question Stats:
59% (01:16) correct 41% (01:13) wrong based on 396 sessions
HideShow timer Statistics
How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9 m12 #19 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process.
Official Answer and Stats are available only to registered users. Register/ Login.




Math Expert
Joined: 02 Sep 2009
Posts: 59020

Re: m12 #19 How many of the factors of 72
[#permalink]
Show Tags
Updated on: 09 Sep 2010, 00:11
gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\). Answer: E. Hope it helps.
_________________
Originally posted by Bunuel on 08 Sep 2010, 23:09.
Last edited by Bunuel on 09 Sep 2010, 00:11, edited 1 time in total.




Intern
Joined: 08 Oct 2012
Posts: 26

How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
19 Nov 2012, 22:21
How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9
I got it right, but I would like to know if my method is efficient.
72 = 2*2*2*3*3 therefore, different numbers that can be found from the above = 5!/3!*2! = 10 Out of these 10, only one number (3*3) is odd....hence, the answer is 101 = 9.



VP
Joined: 23 Mar 2011
Posts: 1098
Concentration: Healthcare, Strategy

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
19 Nov 2012, 23:01
kapsycumm wrote: How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9
I got it right, but I would like to know if my method is efficient.
72 = 2*2*2*3*3 therefore, different numbers that can be found from the above = 5!/3!*2! = 10 Out of these 10, only one number (3*3) is odd....hence, the answer is 101 = 9. you were lucky there I'm afraid the method is incorrect. Sol: The number of factors of 72 will be 12 and not 10. The best way to find out is if X= a^b * c^d then number of factors are (b+1) * (d+1) here 72= 2^3*3^2 so number of factors will be (3+1) * (2+1) = 12 > this includes 1 and the number itself. so for finding factors not divisible by 2, remove all the 2s from the prime factorization. You will be left with 3^2. so factors will be 3 and 3^2(=9). Also, we need to include the number 1 to this list, as it is odd and not divisible by 2. so 123=9 the flaw in your approach: 1.) 5!/2!*3! will give you the number of ways you can arrange (permute) 22233. essentially it gives you the following list: 22233,22323,33222,32322 etc. As you can see this is a mere representation of how you can write three 2s and two 3s. This does NOT give you the list of factors of 72. 2.) not only 3*3 is odd but 3 and 3*3 both are odd factors. Include 1 to this list and you get the 3 odd factors. hope this helps.



Intern
Joined: 20 Oct 2012
Posts: 5

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
19 Nov 2012, 23:19
kapsycumm wrote: How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9
I got it right, but I would like to know if my method is efficient.
72 = 2*2*2*3*3 therefore, different numbers that can be found from the above = 5!/3!*2! = 10 Out of these 10, only one number (3*3) is odd....hence, the answer is 101 = 9. I'm with the poster above  think you were lucky on this one. A quick check (good thing listing factors of 72 doesn't take long): Factors  (12) 1, 2, 3, 4, 6, 8, 9, 12, 18, 24, 36, 72 Factors NOT divisible by 2  (3) 1, 3, 9 Therefore answer is 12  3 = 9.



Intern
Joined: 20 Oct 2012
Posts: 5

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
19 Nov 2012, 23:27
jumsumtak wrote: kapsycumm wrote: How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9
I got it right, but I would like to know if my method is efficient.
72 = 2*2*2*3*3 therefore, different numbers that can be found from the above = 5!/3!*2! = 10 Out of these 10, only one number (3*3) is odd....hence, the answer is 101 = 9. you were lucky there I'm afraid the method is incorrect. Sol: The number of factors of 72 will be 12 and not 10. The best way to find out is if X= a^b * c^d then number of factors are (b+1) * (d+1) here 72= 2^3*3^2 so number of factors will be (3+1) * (2+1) = 12 > this includes 1 and the number itself. so for finding factors not divisible by 2, remove all the 2s from the prime factorization. You will be left with 3^2. so factors will be 3 and 3^2(=9). Also, we need to include the number 1 to this list, as it is odd and not divisible by 2. so 123=9 the flaw in your approach: 1.) 5!/2!*3! will give you the number of ways you can arrange (permute) 22233. essentially it gives you the following list: 22233,22323,33222,32322 etc. As you can see this is a mere representation of how you can write three 2s and two 3s. This does NOT give you the list of factors of 72. 2.) not only 3*3 is odd but 3 and 3*3 both are odd factors. Include 1 to this list and you get the 3 odd factors. hope this helps. BTW  jumsumtak actually shows you how to find the number of factors for a number, and will be a time saver in the exam. Note the example here works with only two prime factors. For another example with three prime factors: "How many factors in 360?", 360 = 5 x 8 x 9 > (1+1) x (3+1) x (2+1) = 24.



VP
Joined: 23 Mar 2011
Posts: 1098
Concentration: Healthcare, Strategy

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
19 Nov 2012, 23:59
jcaine wrote: BTW  jumsumtak actually shows you how to find the number of factors for a number, and will be a time saver in the exam. Note the example here works with only two prime factors. For another example with three prime factors: "How many factors in 360?", 360 = 5 x 8 x 9 > (1+1) x (3+1) x (2+1) = 24.
That is correct. This works with every number not with just 2 prime factors. you can have 'n' PRIME factors and it will still hold true. 360 = 5 x 8 x 9 = 5 x 2^3 x 3^2. So the factors will be (1+1) x (3+1) x ( 2+1) = 2 x 4 x 3= 24



Manager
Joined: 05 Nov 2012
Posts: 138

Re: m12 #19 How many of the factors of 72
[#permalink]
Show Tags
20 Nov 2012, 08:03
Bunuel wrote: gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\). Answer: E. Hope it helps. number of factors and prime factors is fine.... but out of those number of factors... how did you conclude on as only 3 being odd?



Intern
Joined: 20 Oct 2012
Posts: 5

Re: m12 #19 How many of the factors of 72
[#permalink]
Show Tags
24 Nov 2012, 21:20
Amateur wrote: number of factors and prime factors is fine.... but out of those number of factors... how did you conclude on as only 3 being odd? (Will try to explain this using an easier but slightly slower approach since many have difficulties grasping perms & combs) Using the number 72 from the original question; [1] Find number of factors: 1. 72 = 2^3 x 3^2; 2. therefore number of factors = (3+1) x (2+1) = 12 [2] Find number of ODD factors: *Number property, N1: We know that all primes, except 2, are odd. *Number property, N2: We know that ODD x ODD = ODD. *Number property, N3: Multiplying any number by 2 (an Even Number) will yield an EVEN number. 1. Recalling from [1], we have identified 2 and 3 as the prime factors of 72. 2. We ignore the "2" remembering N3. 3. We can construct factors that consist of prime factor 3 only:3, 3 x 3 (since there are only two "3"s we stop here). 4. Let's not forget that "1" is also a noneven factor. 5. Total of ODD factors is 3. [3] Find number of EVEN factors: Total of factors  total of odd factors = total of even factors = 12  3 = 9. One could directly use combinations of 2, 2, 2, 3, 3 to list all EVEN factors but I've found it faster to find ODD factors first. For example, in my explanation for counting factors for a number with three distinct primes: 1. 360 = 5 x 8 x 9 = 5^1 x 2^3 x 3^2 2. Number of primes = (1+1) x (3+1) x (2+1) = 24 3. Number of odd factors will be multiples of only up to 1 "5" and 2 "3"s. 4. List odd factors: 3 5 9 = 3 x 3 15 = 3 x 5 45 = 3 x 3 x 5 5. Do not forget that "1" is also a factor, therefore there are 6 ODD factors in 360. 6. Total of EVEN factors in 360 is 24  6 = 18. *Quick Check with pairs indeed reveals 6 ODD factors: 1, 360 > 1 is ODD 2, 180 3, 120 > 3 is ODD 4, 90 5, 72 > 5 is ODD 6, 60 8, 45 > 45 is ODD 9, 40 > 9 is ODD 10, 36 12, 30 15, 24 > 15 is ODD 18, 20 Hope this clarifies things. * Do note that I would expect 750+ questions to involve combinatorics that involve the use of perms & combs to solve within the time limit.



Manager
Joined: 14 Jan 2013
Posts: 130
Concentration: Strategy, Technology
GMAT Date: 08012013
GPA: 3.7
WE: Consulting (Consulting)

Re: m12 #19 How many of the factors of 72
[#permalink]
Show Tags
07 Mar 2014, 20:27
Bunuel wrote: gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\). Answer: E. Hope it helps. Bunuel, how do we know the red part without writing all the factors of 72?



Math Expert
Joined: 02 Sep 2009
Posts: 59020

Re: m12 #19 How many of the factors of 72
[#permalink]
Show Tags
08 Mar 2014, 06:45
Mountain14 wrote: Bunuel wrote: gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\).Answer: E. Hope it helps. Bunuel, how do we know the red part without writing all the factors of 72? It's not hard to find the number of odd factors of 72 manually but if you want more systematic approach, refer to the red part above or consider the following: Get rid of all the 2’s which give even factors in 72, so divide 72 by 2^3=8: 72/2^3=9=3^2. Now, 9 will have all the odd factors of 72 and won’t have its even factors. The number of factors of 9 is (2+1)=3. So, we know that 72 has total of 12 factors out of which 3 are odd. Therefore 72 has 123=9 even factors. Hope it's clear.
_________________



Manager
Joined: 14 Jan 2013
Posts: 130
Concentration: Strategy, Technology
GMAT Date: 08012013
GPA: 3.7
WE: Consulting (Consulting)

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
10 Mar 2014, 05:06
Yes , it clear... thanks



Manager
Joined: 13 Aug 2012
Posts: 90

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
25 Mar 2014, 09:03
ANOTHER METHOD Just count the no of different factors 72 has
1x72 2x36 3x24 4x18 6x12 9x8 Now of of these  2,4,6,72,36,24,18,12 and 8 are factors divisible by 2 i.e a total of 9 factors. Did not take more than a minute.



Director
Joined: 02 Sep 2016
Posts: 643

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
28 Mar 2017, 09:46
Total no. of factors= (a+1).(b+1). and so on These factors will include all the even factors, odd factors including 1, and the number itself. Easier method can be: Total no. of factorsodd factors 72= 2^3. 3^2 Here a=3 and b=2 Therefore total no. of factors= (3+1).(2+1)=12 Total no. of odd factors (easy!! don't count power of 2)= 3 Thus the total no. of even factors are 123= 9
_________________
Help me make my explanation better by providing a logical feedback.
If you liked the post, HIT KUDOS !!
Don't quit.............Do it.



Intern
Joined: 11 Dec 2016
Posts: 47

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
29 May 2018, 03:03
Bunuel wrote: gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\). Answer: E. Hope it helps. BunuelIn the highlighted part why cant we have 2 in the power of 0 as we have 3 in the power of 0?



Math Expert
Joined: 02 Sep 2009
Posts: 59020

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
29 May 2018, 07:47
asfandabid wrote: Bunuel wrote: gtr022001 wrote: How many of the factors of 72 are divisible by 2? a. 4 b. 5 c. 6 d. 8 e. 9 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. FIRST:Finding the Number of Factors of an IntegerFirst make prime factorization of an integer \(n=a^p*b^q*c^r\), where \(a\), \(b\), and \(c\) are prime factors of \(n\) and \(p\), \(q\), and \(r\) are their powers. The number of factors of \(n\) will be expressed by the formula \((p+1)(q+1)(r+1)\). NOTE: this will include 1 and n itself. Example: Finding the number of all factors of 450: \(450=2^1*3^2*5^2\) Total number of factors of 450 including 1 and 450 itself is \((1+1)*(2+1)*(2+1)=2*3*3=18\) factors. BACK TO THE ORIGINAL QUESTION:According to the above as \(72=2^3*3^2\), then # of factors of 72 is \((3+1)(2+1)=12\). Out of which only 3 are odd 1, 3, and 9, so rest or 123=9 are even. OR: as \(72=2^3*3^2\) then even factors MUST have 2 either in power of 1, 2, or 3 so 3 options and 3 either in power 0, 1, or 2 again 3 options > \(3*3=9\). Answer: E. Hope it helps. BunuelIn the highlighted part why cant we have 2 in the power of 0 as we have 3 in the power of 0? We are counting EVEN factors, so factors which have 2 in them. If 2 were in power of 0, the factor won't be even anymore (2^0 = 1).
_________________



Director
Joined: 12 Feb 2015
Posts: 933

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
05 Jun 2018, 22:03
ANOTHER METHOD:72 can be written as 2*36 To find the number of factors of 36 is a straightforward application of number of factors formula: (p+1)(q+1)(r+1)... [where p,q,r are exponents of each prime factor] Therefore 36 can be written as \(36=2^2*3^2\) Therefore the number of factors of 36 are (2+1)(2+1) = 9 factors. Therefore the no. of factors of 72 which are divisible by 2 are 9 factors. Ans: Option E is correct!!
_________________
________________ Manish "Only I can change my life. No one can do it for me"



ISB School Moderator
Joined: 08 Dec 2013
Posts: 604
Location: India
Concentration: Nonprofit, Sustainability
WE: Operations (NonProfit and Government)

Re: How many of the factors of 72 are divisible by 2?
[#permalink]
Show Tags
27 Jun 2019, 07:39
gtr022001 wrote: How many of the factors of 72 are divisible by 2? A. 4 B. 5 C. 6 D. 8 E. 9m12 #19 What is the quickest and fastest way to find all factors of 72? I drew a prime factor tree but missed some factors in the process. 72= 2^3 * 3^2 Total# Factors: 4*3 Total# Factors where power of 2 is atleast one: 3*3= 9
_________________
Kindly drop a '+1 Kudos' if you find this post helpful.GMAT Math Book I never wanted what I gave up I never gave up what I wanted




Re: How many of the factors of 72 are divisible by 2?
[#permalink]
27 Jun 2019, 07:39






