Last visit was: 12 Dec 2024, 01:52 It is currently 12 Dec 2024, 01:52
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
pzazz12
Joined: 22 Sep 2010
Last visit: 25 Oct 2010
Posts: 59
Own Kudos:
1,125
 [8]
Posts: 59
Kudos: 1,125
 [8]
1
Kudos
Add Kudos
7
Bookmarks
Bookmark this Post
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 11 Dec 2024
Posts: 97,828
Own Kudos:
Given Kudos: 88,254
Products:
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 97,828
Kudos: 685,205
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
shrouded1
User avatar
Retired Moderator
Joined: 02 Sep 2010
Last visit: 29 Apr 2018
Posts: 611
Own Kudos:
3,030
 [1]
Given Kudos: 25
Location: London
 Q51  V41
Products:
Posts: 611
Kudos: 3,030
 [1]
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 11 Dec 2024
Posts: 97,828
Own Kudos:
685,205
 [1]
Given Kudos: 88,254
Products:
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 97,828
Kudos: 685,205
 [1]
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
shrouded1
pzazz12
A number N when expressed as product of prime factors gives N= 2^5 * 3^2* 5^4 * 7 * 11^3. Find the number of ways the number N can be expressed as a product of three factors such that the factors are pairwise co-prime.

A.1027
B.3125
C.243
D.729
E.None of these

The trick is treat \(2^5, 3^2, 5^4, 7, 11^3\) as 5 objects which you need to place in 3 groups. Order being irrelevant as that cannot matter

So the answer would be :
\(\frac{5!}{0!0!5!} + \frac{5!}{1!0!4!} + \frac{5!}{1!1!3!} + \frac{5!}{2!0!3!} + \frac{5!}{2!1!2!}\)
\(= 1 + 5 + 20 + 10 + 30 =66\)

Now its been too long since I learnt P&C, and this is a hard question ... so I am not 100% confident, but I would guess (e)

What I am 100% confident about is that this is beyond a GMAT difficultly level

I think 66 contains duplication.

For example if we manually count case 1-1-3 we will get:
{2}-{3}-{5, 7, 11}
{2}-{5}-{3, 7, 11}
{2}-{7}-{3, 5, 11}
{2}-{11}-{3, 5, 7}
{5}-{3}-{2, 7, 11}
{7}-{3}-{2, 5, 11}
{11}-{3}-{2, 5, 7}
{5}-{7}-{2, 3, 11}
{5}-{11}-{2, 3, 7}
{7}-{11}-{2, 3, 5}

Total of 10 cases (when order is not important) but as per your formula it's \(\frac{5!}{1!1!3!}=20\). The same with 1-2-2, it's also twice as much. If you subtract this duplications 10 and 15 then you'll get 41 as in my calculations.
User avatar
shrouded1
User avatar
Retired Moderator
Joined: 02 Sep 2010
Last visit: 29 Apr 2018
Posts: 611
Own Kudos:
Given Kudos: 25
Location: London
 Q51  V41
Products:
Posts: 611
Kudos: 3,030
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Good spot ! Agreed.

I can intuitively tell why the duplication exists. Its sort of because the two "equal" groups in 3-1-1 and 2-2-1 are somehow treated as "distint" in these formulae. I agree with the 41. Surely order can't metter though
User avatar
OldFritz
Joined: 15 Sep 2009
Last visit: 29 Sep 2020
Posts: 132
Own Kudos:
Given Kudos: 6
Posts: 132
Kudos: 121
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Irrelevant practice for the GMAT but I am sure it's fun for those who solved it.
User avatar
vitaliyGMAT
Joined: 13 Oct 2016
Last visit: 26 Jul 2017
Posts: 299
Own Kudos:
807
 [1]
Given Kudos: 40
GPA: 3.98
Posts: 299
Kudos: 807
 [1]
Kudos
Add Kudos
1
Bookmarks
Bookmark this Post
pzazz12
A number N when expressed as product of prime factors gives N= 2^5 * 3^2* 5^4 * 7 * 11^3. Find the number of ways the number N can be expressed as a product of three factors such that the factors are pairwise co-prime.

A.1027
B.3125
C.243
D.729
E.None of these

“Pairwise coprime” means that if we choose any 2 numbers from a given set of numbers they will be co-prime to each other.

This question is about splitting into groups*.

We have 5 possible cases including those which have 1 as a factor:

1*1*(group of 5 primes)
1*(group of 1 prime)(group of 4 primes)
1*(group of 2 primes) (group of 3 primes)
(group of 1 prime)*(group of 1 prime)*(group of 3 primes)
(group of 1prime)*(group of 2 primes)*(group of 2 primes)

* - in each group primes are different and choosing 1 as a factor means choosing 0 from a group of a given 5 different prime factors.

Values of given answer options suggest that order of factors matters (we won’t be able to achieve such big numbers otherwise).

Hence, we have:

\(\frac{5!}{0!*0!*5!} * \frac{3!}{2!} + \frac{5!}{0!*1!*4!} *3! + \frac{5!}{0!*2!*3!} *3! + \frac{5!}{1!*1!*3!} *\frac{3!}{2!} + \frac{5!}{1!*2!*2!} * \frac{3!}{2!} =\)

\(= 3 + 30 + 60 + 60 + 90 = 243\)
Moderator:
Math Expert
97827 posts