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GMAT Club > Tests > m01 > Q.35

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Re: GMAT Club > Tests > m01 > Q.35 [#permalink]

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New post 02 Jan 2010, 13:48
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Re: GMAT Club > Tests > m01 > Q.35 [#permalink]

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The symbol is just a symbol, it means nothing. In the question it denotes some function relationship.

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.

@@90 --> \(90=2*3^2*5\) the number of distinct factors (divisors) of 90 can be found by the formula: (1+1)(2+1)(1+1)=12. So @90=12 --> @12=? 12=2^2*3, again the number of distinct factors (divisors) of 12 can be found by the formula: (2+1)(1+1)=6.

Answer: D (6)

General rule:

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.

This question is about the number properties. You can check the Number Theory chapter in MathBook (link below), sorry it's not finished yet.
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Re: GMAT Club > Tests > m01 > Q.35 [#permalink]

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New post 02 Jan 2010, 14:33
Great explanation ! Never came across this formula before. Another feather in the cap. :) Thanks !

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Re: GMAT Club > Tests > m01 > Q.35   [#permalink] 02 Jan 2010, 14:33
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