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Re: A number N when expressed as product of prime factors gives. [#permalink]
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shrouded1 wrote:
pzazz12 wrote:
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.
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Re: A number N when expressed as product of prime factors gives. [#permalink]
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
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Re: A number N when expressed as product of prime factors gives. [#permalink]
Irrelevant practice for the GMAT but I am sure it's fun for those who solved it.
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Re: A number N when expressed as product of prime factors gives. [#permalink]
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pzazz12 wrote:
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:

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

\(= 3 + 30 + 60 + 60 + 90 = 243\)
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Re: A number N when expressed as product of prime factors gives. [#permalink]
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