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Re: If @x is the number of distinct positive divisors of x, what is the va [#permalink]
28 Feb 2008, 13:46

pmenon wrote:

is there a quick way to find all the factors of 90, or do you just have to go through all the numbers ?

You can find out all the prime factors. But you probably wont save too much time here. might be a better approach on bigger numbers though.

2,3,3,5 dont forget 1 at the end. multiply all the possible combinations by.

Ex/ 2*3, 3*3, etc...

I cannot come up with an actual combinatorics solution yet, but im done w/ my work for the day and got bout 20min til i go home so il try and see if i can think of something...

Re: If @x is the number of distinct positive divisors of x, what is the va [#permalink]
29 Feb 2008, 08:13

I got D a different way. I found the primes of 90, distinct, which came out to 2,3,5. Since there are two OO, i multiplyed 3 * 2 = 6. Did I do it correctly?

Re: If @x is the number of distinct positive divisors of x, what is the va [#permalink]
27 Sep 2014, 04:52

terp26 wrote:

The quick way to get # of factors is to find prime factor the number:

90 = 3^2 * 5^1 * 2^1

Then you add the number 1 to exponent and multiply exponents together.

so (2+1) (1+1) (1+1) = 3*2*2 = 12

12 is number of factors of 90

do the same to get number of factors of 12

12 = 2^2 * 3^1

(2+1) * (1+1) = 6

Is this how to calculate the number of distinct factors? In the number 12, we have 2,3,4,6... but 6 is made up from 2 and 3, so does it count in the distinct? what about 216? it's 6*6*6... so does 6*6 count as distinct, or do we just count 6? I got confused here....

Re: If @x is the number of distinct positive divisors of x, what is the va [#permalink]
27 Sep 2014, 05:00

Expert's post

ronr34 wrote:

terp26 wrote:

The quick way to get # of factors is to find prime factor the number:

90 = 3^2 * 5^1 * 2^1

Then you add the number 1 to exponent and multiply exponents together.

so (2+1) (1+1) (1+1) = 3*2*2 = 12

12 is number of factors of 90

do the same to get number of factors of 12

12 = 2^2 * 3^1

(2+1) * (1+1) = 6

Is this how to calculate the number of distinct factors? In the number 12, we have 2,3,4,6... but 6 is made up from 2 and 3, so does it count in the distinct? what about 216? it's 6*6*6... so does 6*6 count as distinct, or do we just count 6? I got confused here....

First of all, the factors of 12 are 1, 2, 3, 4, 6, and 12. Next, how is 6 and 6 different from one another? You should count it once.

If @x is the number of distinct positive divisors of x, what is the value of @(@90)?

A. 3 B. 4 C. 5 D. 6 E. 7

Finding the Number of Factors of an Integer

First 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:

The question defines @x as the number of distinct positive divisors of x. Say @6=4, as 6 have 4 distinct positive divisors: 1, 2, 3, 6.

Question: @(@90)=?

90=2*3^2*5, which means that the number of factors of 90 is: (1+1)(2+1)(1+1)=12. So @90=12. Next, @(@90)=@12. Now, since 12=2^2*3, then the number of factors of 12 is: (2+1)(1+1)=6.